Chroma-key compositing is a way to make replacing parts of a video with something else easier. Usually a color that is not present in a scene will be selected and any parts that are to be replaced will be covered or painted in that color. Later, in post-production, the color is replaced with other imagery. The selected color is the called the chroma-key.
This technique is often called green-screen or blue-screen because it is frequently used to replace the background so that it looks like the actors are somewhere else. It is also often used by having an actor wear a tight, chroma-key-colored outfit so that they can be removed entirely.