In regular motocross, riders often catch air while going over hills on the race course. It's an added bonus for fans but not the main point of the race. In freestyle motocross, catching air is the point.
To win in freestyle motocross, riders launch their bikes over jumps and perform intricate tricks while airborne. Stunts include the can can, where riders put both legs on the same side of the bike (extra points for landing in the position), and the lazy boy, where riders put their feet up beyond the handle bars and lay back on the bike with their hands extended behind their head (that doesn't sound very lazy at all, actually).
The more impressive and well-executed the tricks are, the better score the judges give the rider. In this way, freestyle motocross is like gymnastics — only instead of acrobats flipping on mats and parallel bars, there are riders covered in mud hanging on to a gas-powered machine as they fly several stories above a dirt track. See? Exactly the same.