If you spin on the spot and go completely around just once and stop, you "did a 360" since you moved in a circle and circles have 360 degrees.
So a 720 would be two complete turns and a 540 would be 1 and a half complete turns.
The number is the amount of degrees they rotated through. What's cool is, snow boarders do this while in mid air as a trick when going over jumps.
