The Quran tells us in detail about how God created Adam. Many of you would know the story. When God created Adam, two other creatures were present at that time: angels and Satan. God ordered both angels and Satan to bow before Adam. Angels bowed down but Satan refused. Then a dialogue occurred between God and Satan, which you can read in the Quran. In this dialogue, Satan presented a challenge. 
This challenge was: Copy Arabic text (7:17) This was Satan's challenge, as mentioned in the Quran. Satan told God that he would lead humans astray, leaving very few to be truly grateful. Wa la tajid aktharahum shakireen means I will turn most of them ungrateful. I will turn the majority of humans ungrateful. This was the challenge presented by Satan. What does it mean to be grateful and what does it mean to be ungrateful? In reality, this is the test of this life. 
What is shukr?
Shukr means to acknowledge. Shukr isn't anything mysterious. Shukr means to acknowledge God. God has created us, He gave us health, He gave us hands and legs, eyes and everything else. I was thinking God has given us billions and trillions of blessings in the form of this life, innumerable human beings, family, friends, life support system, solar system and so on. If God were to take away only one blessing, for example, these eyes, if every person turned blind: I have read about many people who slept at night and woke up blind the next day. I have seen a blind man whose eyes were functioning properly earlier. When he slept at night, his eyes were fine. But when he woke up the next day, he had become blind. This man worked at Germany's Indian embassy. He was blind from both eyes. I have seen many such persons who were earlier fine but later became blind. If God were to turn every human being blind, everything would come to an end. When you can't see anything, you would collide into walls, or animals or other people. You will have a car, but won't be able to drive it. You will have a house but living in it would become impossible. You will have a kitchen but you won't be able to cook in it. The entire civilization would come to a halt. All the splendour you see in cities would suddenly fade away. The activities of our homes, cities, civilization would abruptly come to an end, if only our eyes were taken away.  This is the importance of God's blessing. 
God's blessings are so innumerable, we cannot count them: Copy Arabic text (16:18) If you tried to count God's blessings, you would never be able to number them. 

These blessings have been unilaterally given by God. God doesn’t send us any bills. We have to acknowledge these blessings. We must thank God from the core of our hearts for everything: our eyes, legs, the world, solar system and so on. We need to discover this world as a blessing and acknowledge God for it. This is shukr.