Gravity is the force that pulls objects to Earth, like playing basketball. Anything which has mass also has a gravitational pull. The more massive/bigger the object is, the stronger its gravitational pull is.
Michael is about to make his shot, but in order to have a better chance of it going in, Michael jumps really high up the get the height that he needs. The gravity is pulling him to the ground though so he can't stay in the air for very long.