Definition - What does Chroma Key mean?
Techopedia explains Chroma Key
Chroma keying is used in movies to replace a blue or green background with computer-generated or separately-shot scenes during the post-production stage. The selection of blue or green back ground mainly depends on the effect required and what colors the actors are wearing. This makes it easier to separate the actor from the background. The final result is that the film is made to look like the actor is somewhere other than a studio.
The chroma key process is widely used because it is cheaper to do this than to shoot in expensive or inaccessible locations. It can also be done in real time, making it ideal for weather report or entertainment shows.