Compositing is the ability to combine multiple layers of video in a single screen and have them interact with one another. This capability adds great depth to FCP. Compositing allows you to create a montage of images and graphics that can enhance a mundane portion of a production.
Good compositing work can raise the perceived quality of a production and is used for a great deal of video production work on television—commercials, of course—but also on news programs and for interstitials, the short videos that appear between sections of a program. Be warned, though, that compositing and graphics animation are not quick and easy to do. Most compositing is animated, and animation requires patience and skill.