Time_step is used for maintaining the games speed at all frame rates what it is saying is that if you have a frame rate of 30 and a frame rate of 230 you can include time_step in your movement portions of the code and it will move at the same speed. It does this by measuring the time of the last frame and adjusts its self accordingly. Say you have a fps of 15 and ur using time_step.then your fps goes up too 100, time_step will decrease its value so the movement maintains the same speed. That's basically it without using all the math they show u in the tutorial.