Chroma keying is a video editing process more commonly known as "green screening." The term "green screen" refers to the colored background used during video recording that is later removed from the shot and replaced with a different background. This is usually a single colored backdrop, which can be any color, but is usually bright green because it is the color furthest away from human skin tones (blue screens were frequently used in the early days with film, and might still be used in certain cases). Chroma keying is the actual technique of layering, or compositing, two videos based on color hues (i.e. green hues in the background) in post-production using video editing software. When the green screen background has been keyed, it will be fully transparent. Then you can fill in that transparent area with a different image or video.
Want to learn how to use green screens for chroma keying? Check out the Premiere Pro training video from Udemy (available to NAU affiliates).