Chromakey static is a "best-of-class compositing ('blue-', 'green-screening') plug" for animated/static source combinations.
But what does it do? Simply put, it allows you to intelligently blend a picture into a movie or vice versa. This is more complex than creating a simple mask because a clip can change it's contents rapidly, and the mask has to adapt. Chromakey static allows you to target any color in your picture or movie and create a transparency mask from that color. The best known example for this technique is the weatherman in front of his weather chart. The process of intelligently combining a dynamic and a static source is also known as Œcompositing‚. the technique of replacing a key color with a whole picture is sometimes called Œchroma keying‚.