What Is Qadar in Islam?
Question

What is the meaning of belief in al-Qadar (the divine will and decree)?
Praise be to Allah.What is Qadar?
Qadar means that Allah has decreed everything that happens in the universe according to His prior knowledge and the dictates of His wisdom. 
Belief in al-Qadar includes four things: 

The belief that Allah knows all things, in general and in detail, from eternity to eternity, whether that has to do with His actions or the actions of His slaves. 
The belief that Allah has written that in al-Lawh al-Mahfuz (the Book of Decrees). 

Concerning these two matters Allah says (interpretation of the meaning): 
“Know you not that Allah knows all that is in the heaven and on the earth? Verily, it is (all) in the Book (Al-Lawh Al-Mahfuz). Verily, that is easy for Allah.” [al-Hajj 22:70]
In Sahih Muslim (2653), it is narrated that ‘Abd-Allah ibn ‘Amr ibn al-‘As (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: “Allah wrote down the decrees of creation fifty thousand years before He created the heavens and the earth.”  
And the Prophet (peace and blessings of Allah be upon him) said: “The first thing that Allah created was the Pen, and He said to it, ‘Write!’ It said, ‘O Lord, what should I write?’ He said: ‘Write down the decrees of all things until the Hour begins.” Narrated by Abu Dawud, 4700; classed as sahih by al-Albani in Sahih Abi Dawud. 

The belief that whatever happens only happens by the will of Allah – whether that has to do with His actions or the actions of created beings. 

Allah says concerning His actions (interpretation of the meaning): 
“And your Lord creates whatsoever He wills and chooses.” [al-Qasas 28:68] 
“and Allah does what He wills.” [Ibrahim 14:27] 
“He it is Who shapes you in the wombs as He wills.” [Aal ‘Imran 3:6]
And He says concerning the actions of created beings (interpretation of the meaning): 
“Had Allah willed, indeed He would have given them power over you, and they would have fought you.” [al-Nisa 4:90] 
“If your Lord had so willed, they would not have done it.” [al-An’am 6:112]
All events, actions and happenings occur only by the will of Allah. Whatever Allah wills happens, and whatever He does not will does not happen.  

The belief that all things that happen are created by Allah in their essence, their attributes and their movements. 

Allah says (interpretation of the meaning): 
“Allah is the Creator of all things, and He is the Wakil (Trustee, Disposer of affairs, Guardian) over all things.” [al-Zumar 39:62] 
“He has created everything, and has measured it exactly according to its due measurements” [al-Furqan 25:2]
And Allah tells us that the Prophet of Allah Ibrahim (peace and blessings of Allah be upon him) said to his people: 
“While Allah has created you and what you make.” [al-Saffat 37:96 – interpretation of the meaning]
If a person believes in all these things then he believes correctly in the divine will and decree (al-qadar). 
Belief in al-qadar and free will
Belief in al-qadar as we have described it above does not contradict the idea that a person has free will with regard to actions in which he has free choice. He can choose whether to or not to do things that he is able to do of acts of worship or sinful actions. Shari’ah and real life both indicate that people have this will. 
With regard to shari’ah, Allah says concerning man's will (interpretation of the meaning): 
“That is (without doubt) the True Day. So, whosoever wills, let him seek a place with (or a way to) His Lord (by obeying Him in this worldly life)!.” [al-Naba 78:39] 
“so go to your tilth, when or how you will.” [al-Baqarah 2:223]
And He says concerning man’s ability (interpretation of the meaning): 
“So keep your duty to Allah and fear Him as much as you can.” [al-Taghabun 64:16] 
“Allah burdens not a person beyond his scope. He gets reward for that (good) which he has earned, and he is punished for that (evil) which he has earned.” [al-Baqarah 2:286]
These verses confirm that man has a will and the ability to do what he wants and not to do what he does not want.  
With regard to real life, everyone knows that he has a will and the ability to do what he wants and not to do what he does not want. And he can distinguish between the things that happen when he wants them to, such as walking, and those that happen without him wanting them to, such as shivering. But the will and ability of man are subject to the will and decree of Allah, because Allah says (interpretation of the meaning): 
“To whomsoever among you who wills to walk straight.  And you cannot will unless (it be) that Allah wills the Lord of the ‘Alamin (mankind, jinn and all that exists).” [al-Takwir 81:28-29]
But the entire universe is the dominion of Allah, and nothing can happen in His dominion without His knowledge and will. 
And Allah knows best. 
Reference:
Risalat Sharh Usul al-Iman by Shaykh Ibn ‘Uthaymin.