Frisbee is a game of throw and catch. What happens between the throw and catch are where most tricks take place. Air-brushing is one of the base tricks that leads into many other tricks.
The Air-Brush is act of striking or swiping a disc in flight on the outer rim in the same direction as the spin. The goal is to keep the disc in flight, redirect the flight, and / or to add spin. When multiple air-brushes are performed successively it can look a little like juggling.
These articles will help get you started with air-brushing.