Chromakey movie is a "best-of-class compositing ('blue-', 'green-screening') plugin" for iMovie. But what does it do? Simply put, it allows you to intelligently blend two movies dynamically. This is more than applying a simple transparency mask (as our plug 'simple mask' does) because it allows you to target any color in one of your movies and make that color transparent.
This mask is re-calculated dynamically for every frame in your clip. The best known example for this technique is a newscaster who seems to be standing in front of the white house.