It's the effect used by weatherperson on TV. Put solid color screen behind object your taping, and chroma key it out. Easy to use, but tricky getting great results. Put another video on higher track, for replacing keyed out color.
Learn to use Masking (in Event Pan/Crop) also. It helps make up for poor job of blue/green/red or whatever solid color you use to keying out. AND it's fun to play with.
One simple example would be to tape skateboarding on a treadmill with blue screen background. Make second tape of anything you want to "look like" you're boarding on. Limitations are imagination, planning, editing, and probably lighting.