Meaning of Lowering the Gaze
Question

What is meant by lowering the gaze?
Praise be to Allah.Linguistic meaning of lowering the gaze
Lowering the gaze (Ghadd Al-basar) means restraining the gaze and not allowing it to wander or dwell upon anything.
Ibn Faris said in Mu`jam Maqayis Al-Lughah (4/307): “Ghayn and dad indicate restraining, as in the phrase Ghadd Al-basar (lowering the gaze)…”
Ibn Al-Manzur said in Lisan Al-`Arab (7/196): “Lowering the gaze (Ghadd Al-basr) means restraining it.”
The Islamic meaning of lowering the gaze
In Islamic terminology lowering the gaze refers to a number of things:

Refraining from looking at people’s `Awrahs, which includes the beauty of a non-mahram woman.

Shaykh Al-Islam Ibn Taymiyah (may Allah have mercy on him) said in Majmu` Al-Fatawa (15/414):
“Allah, may He be Glorified and Exalted, has enjoined us in His Book to lower the gaze, which is of two types: refraining from looking at `Awrahs and refraining from looking at the site of desire.
The former refers to a man refraining from looking at the `Awrah of another person.
The second refers to looking at uncovered parts of a non-mahram woman. This is more serious than the former, just as alcohol is more serious than dead meat and blood and pork, and the hadd punishment should be carried out on the one who drinks it, because these prohibited things are not as desirable as alcohol may be.”

Refraining from looking into people’s houses and things that are behind closed doors.

Ibn Taymiyah (may Allah have mercy on him) says in Majmu` Al-Fatawa (15/379):
Just as lowering the gaze includes not looking at the `Awrahs of other people and other prohibited things, it also includes refraining from looking into people’s houses. A man’s house conceals his body just as his garments conceal him. Allah has mentioned lowering the gaze and guarding one’s private parts after the verse about asking for permission to enter, because the house covers a person just as the clothes on his body do. End quote.
Ibn Al-Qayyim (may Allah have mercy on him) said in Madarij As-Salikin (1/117):
“Prohibited kinds of looking include looking at `Awrahs, which is of two types: the `Awrah behind a garment and the `Awrah behind doors.” (End quote)

Refraining from looking at what people have of wealth, wives, children, worldly goods and so on.

Allah says (interpretation of the meaning):
“Look not with your eyes ambitiously at what We have bestowed on certain classes of them (the disbelievers), nor grieve over them. And lower your wings for the believers (be courteous to the fellow-believers).” [Al-Hijr 15:88]
Ibn Sa`di (may Allah have mercy on him) said in his Tafsir (434):
“This means, do not admire them in such a way that you will distracted with desire for the worldly pleasures enjoyed by those who live in luxury and by which the ignorant are deceived. You should be content with that which Allah has given you of the seven oft-repeated verses and the Holy Quran (cf. Al-Hijr 15:87). (End quote)
He also said (p. 516):
“This means,  do not be impressed and do not look repeatedly admiring worldly pleasures and those who are enjoying them, such as delicious food and drink, fine clothing, beautiful houses and adorned women, for all of that is the adornment of this world in which those who are deceived rejoice. And those who do wrong enjoy it by ignoring the Hereafter. But it will all soon end and cease to be, and those who love it will die, then they will regret when regret will be to no avail, and they will realize the state they are in when the Resurrection begins. Allah has made it a test and a trial, so that it may be known who will be deceived by it and who will be better in deeds. (End quote)
Benefits of lowering the gaze
The scholars have mentioned many benefits of lowering the gaze, including the following:
Ibn Al-Qayyim (may Allah have mercy on him) said in Al-Jawab Al-Kafi (125):
“There are a number of benefits in lowering the gaze:

It is obedience to the command of Allah, which brings happiness to man in this world and in the next. There is nothing more beneficial to a person in this world and in the next than obeying the commands of his Lord, may He be Blessed and Exalted, and those who are happy in this world and the next can only attain that happiness by obeying His commands, and those who are doomed in this world and in the next are only doomed because they ignore His commands.
It prevents the poisoned arrows (of the devil), which may lead to his doom, from reaching his heart.
It creates a heart that is devoted to and focused on Allah. Letting the gaze wander distracts the heart and keeps it far from Allah. There is nothing more harmful to a person than letting his gaze wander, as it creates alienation between a person and his Lord.
It strengthens the heart and brings it peace, just as letting the gaze wander weakens it and makes it sad.
It brings light to the heart, just as letting the gaze wander brings darkness to it. Hence Allah mentioned the verse of light immediately after the command to lower the gaze, as He says (interpretation of the meaning):

{Tell the believing men to lower their gaze (from looking at forbidden things), and protect their private parts…} [An-Nur 24:30]
Then straight after that He says (interpretation of the meaning):
{Allah is the Light of the heavens and the earth. The parable of His Light is as (if there were) a niche and within it a lamp.} [An-Nur 24:35]
i.e., the likeness of His light in the heart of His believing slave who obeys His commands and heeds His prohibitions. If the heart is enlightened blessings will come to it from all sides, but if it is darkened, calamity and evil will come to it from all places. Whatever exists of innovation, misguidance, following whims and desires, avoiding true guidance and turning away from the means of happiness and focusing on the means that lead to doom, that will be recognizable by means of the light that is in the heart. If that light is lost then one will left like a blind man stumbling in the darkness.

It generates true insight which can distinguish between truth and falsehood, sincerity and lies. Allah rewards His slave for his good deeds with something similar and if he gives up something for the sake of Allah, He will compensate him with something better than it. So if he lowers his gaze and refrains from looking at things that Allah has forbidden, Allah will compensate him with enlightenment; he will compensate him for restraining his gaze for the sake of Allah, and will open to him the doors of knowledge, faith and true insight which he only attained by means of the light in his heart. The opposite of that is the blindness which Allah attributed to the homosexuals, which is the opposite of insight. Allah says (interpretation of the meaning):

{Verily, by your life (O Muhammad (peace and blessings of Allah be upon him), in their wild intoxication, they were wandering blindly.} [Al-Hijr 15:72]

It creates a heart that is steadfast, brave and strong. Allah will give him both insight and power and strength, as it says in the report: “The one who goes against his whims and desires, the devil flees from his shadow.”

On the other hand, the one who follows his whims and desires will feel a sense of humiliation, indignity, worthlessness and insignificance, which is the punishment which Allah has decreed for those who disobey Him as Al-Hasan said: “Even if they ride the finest of mounts, the effect of sin will never depart from them. Allah insists that the one who disobeys Him will be humiliated.”
Allah, may He be Glorified and Exalted, has connected strength to obedience to Him, and humiliation to disobedience to Him. Allah says (interpretation of the meaning):
{But honour, power and glory belong to Allah, and to His Messenger (Muhammad (peace and blessings of Allah be upon him), and to the believers.} [Al-Munafiqun 63:8]
{So do not become weak (against your enemy), nor be sad, and you will be superior (in victory) if you are indeed (true) believers.} [Al `Imran 3:139]
Faith is both words and deeds, inward and outward. Allah says (interpretation of the meaning):
{Whosoever desires honour, power and glory then to Allah belongs all honour, power and glory [and one can get honour, power and glory only by obeying and worshipping Allah (Alone)]. To Him ascend (all) the goodly words, and the righteous deeds exalt it (i.e. the goodly words are not accepted by Allah unless and until they are followed by good deeds)} [Fatir 35:10]
i.e., whoever desires power, let him seek it by means of obedience to Allah and remembrance of Him, by speaking good words and doing good deeds. In Du’a’ Al-Qunut it says: “he is not humiliated whom You have befriended, nor is he honoured who You take as an enemy”. Whoever obeys Allah is His friend in as much as he obeys Him, and he will have support and honour from Him commensurate with his obedience towards Him. Whoever disobeys Him is His enemy in as much as he disobeys Him, and he will have humiliation from Him commensurate with his disobedience towards Him.

It blocks the devil from a means of entering his heart, for he may enter with looking, and penetrate the heart faster than the wind blowing through an empty space, and he may present to him the image that he looked at and make it attractive, like an idol to which his heart becomes devoted, then he encourages him and gives him hopes, and fans the flames of desire in his heart, adding the fuel of sin which could not have reached his heart without looking at that image. So his heart becomes inflamed and surrounded with fire on all sides, resulting in infatuation and frustration, and he is in the midst of it like a lamb in the oven. Hence the punishment for those whose desires were fuelled by prohibited looking is that in Al-Barzakh they are placed in an oven of fire.
It distracts one from thinking of what is in one’s best interests, so his affairs become neglected and he follows his whims and desires and neglects to remember his Lord. Allah says (interpretation of the meaning):

{and let not your eyes overlook them, desiring the pomp and glitter of the life of the world; and obey not him whose heart We have made heedless of Our remembrance, and who follows his own lusts, and whose affair (deeds) has been lost.} [Al-Kahf 18:28]

Between the eyes and the heart there is a connection which means that the one is affected by the other, and if one of them becomes good, the other will also become good, and if one becomes corrupt the other will become corrupt. If the heart becomes corrupt the gaze will become corrupt, and if the gaze becomes corrupt the heart will become corrupt, and similarly if one is sound the other will also be sound.” (End quote)

For more details, please see the following answers: 113996, 126925, 160554, 49024, and 21784.
And Allah knows best.