The idols of the grave worshippers
Question

I would like clarification about what Ibn al-Qayyim (may Allah have mercy on him) meant by the word idol (wathan) in the second line of the poem: “By Allah, they do not get angry when the sacred limits set by Allah are transgressed, secretly or openly, but when something is said about the idol that they call upon, criticizing it, may Allah protect you from their anger and from their fighting you and reviling you, and from their aggression, and may the Most Merciful protect you from their beating, punishment, slander and imprisonment. By Allah, if you were to deny all the divine attributes, they would not react to you with even a part of that aggression.”
Praise be to Allah.These lines of verse occur in a poem quoted by Ibn al-Qayyim (may Allah have mercy on him) when discussing the message of Tawheed (affirmation of Allah’s oneness) which the Messengers were sent to explain, namely Tawheed al-‘Uboodiyyah (oneness of worship, i.e., worshipping Allah alone), so they should not devote any kind of worship – whether it is prayer, fasting, supplication (du‘aa’), vows, pilgrimage, reliance or any other act of worship in the heart or with one’s physical faculties – to anyone other than Allah.
Ibn al-Qayyim (may Allah have mercy on him) pointed out how some of those who claim to be Muslims are misguided, for they have taken some of their shaykhs, or some of those whom they believe to be righteous and pious, as objects of worship. Hence they devote acts of worship to them, by going on pilgrimage to their graves, and offering supplication to them, seeking provision, children and healing, and other types of worship which it is not permissible to devote to anyone other than Allah, may He be exalted. They claim that these occupants of the graves intercede for them with Allah, may He be exalted, and they do the same as the polytheist idol worshippers do, for they base their beliefs on the same specious arguments.
Allah, may He be exalted, said, denouncing such people (interpretation of the meaning):
{Indeed, We have sent down to you the Book, [O Muhammad], in truth. So worship Allah, [being] sincere to Him in religion. 
Unquestionably, for Allah is the pure religion. And those who take protectors besides Him [say], “We only worship them that they may bring us nearer to Allah in position.” Indeed, Allah will judge between them concerning that over which they differ. Indeed, Allah does not guide he who is a liar and [confirmed] disbeliever } [az-Zumar 39:2-3]
{So who is more unjust than he who invents a lie about Allah or denies His signs? Indeed, the criminals will not succeed.
And they worship other than Allah that which neither harms them nor benefits them, and they say, “These are our intercessors with Allah.” Say, “Do you inform Allah of something He does not know in the heavens or on the earth?” Exalted is He and high above what they associate with Him} [Yoonus 10:17-18].
Ibn Katheer (may Allah have mercy on him) said:
Then Allah, may He be exalted, tells us about the polytheists who worship idols, and that they say: “We only worship them that they may bring us nearer to Allah.” In other words, the only thing that made them worship them is that they made idols in the form of angels who are close to Allah, as they claim, then they worshipped those images, believing that this was a way of worshipping those angels, so that they would intercede for them with Allah to support them, provide for them and help them with whatever worldly troubles they faced; as for the hereafter, they denied it and did not believe in it.
Qataadah, as-Saddi and Maalik narrated from Zayd ibn Aslam and Ibn Zayd that “We only worship them that they may bring us nearer to Allah” meant: so that they will intercede for us and bring us closer to Him.
Hence they used to say in their Talbiyah when they performed Hajj during the Jaahiliyyah: “Here we are, You have no partner except a partner whom You have, and You control him and everything he possesses.”
This specious argument is what the polytheists relied on in the ancient past and still rely on it now. The Messengers (blessings and peace of Allah be upon them all) came to refute it and forbid it, and to call people to worship Allah alone, with no partner or associate." (Tafseer Ibn Katheer  7/84-85).
The grave worshippers resemble the polytheists of the Jaahiliyyah, because these graves have become like the idols of the people of the Jaahiliyyah, for the wathan (idol) is a stone that is worshipped.
Ibn Faaris (may Allah have mercy on him) said:
Wathan is the singular of awthaan, and refers to a stone that was worshipped." (Maqaayees al-Lughah  6/85).
Hence whoever devotes any act of worship to anything other than Allah, may He be exalted, whether it is a grave or anything else, has regarded it as a wathan (idol), as it was narrated in the hadith of Abu Hurayrah that the Prophet (blessings and peace of Allah be upon him) said: “O Allah, do not let my grave be turned into an idol (wathan). May Allah curse people who took the graves of their prophets as places of worship.” Narrated by Imam Ahmad in al-Musnad (12/314). Its isnad was regarded as qawiy (strong) by the commentators on al-Musnad.
Ibn ‘Abd al-Barr (may Allah have mercy on him) said:
The word wathan refers to a sanam (idol), which is an image or statue, whether it is made of gold, silver or anything else. Everything that is worshipped besides Allah is a wathan, whether it is an image or not. The Arabs used to pray to idols and worship them, so the Messenger of Allah (blessings and peace of Allah be upon him) feared that his ummah might do the same as some of the previous nations had done: when a prophet of theirs died, they would gather in devotion around his grave as is done with idols." (at-Tamheed  5/45).
Shaykh Muhammad Khaleel Harraas (may Allah have mercy on him) said, commenting on this poetry:
What the poet meant when referring to this group was the groups of those who worship graves and tombs, who claimed falsely to be Muslims.
These grave worshippers regard the occupants of these tombs as partners to Allah; they love them and seek to draw close to them by doing different kinds of acts of worship, such as offering sacrifices and making vows. They love them equally with Allah, may He be glorified and exalted; in fact, they love them more and are more attached to them, as is indicated by their attitude and conduct, for their deeds speak volumes. One of them never gets upset or angry if the sacred limits of Allah are transgressed and He is disobeyed secretly and openly. But if he hears one who truly understands the meaning of Tawheed speaking ill of his idol that he calls upon and to which he devotes his worship, and describing it as helpless and worthless – even if this description is taken from the Qur’an – he gets very angry and shows pride in his worthless beliefs." (Sharh al-Qaseedah an-Nooniyyah 2/139-140).
And Allah knows best.