Definition
Chroma key is a compositing technique that erases an area of a specific color (usually green or blue) in a video or photo, and layers a different background video in its place. The name means using color (chroma) as the key that separates the two.
What happens when a weather forecaster wears green
What would happen if a TV weather forecaster showed up in a green dress? On screen, only her face and arms would float in the air, and a typhoon map or cloud footage would show right through her body.
A computer can't tell a person from a background by eye. Instead, it gets an order from the broadcast equipment: "Wipe out every part of the picture that counts as green." So even the clothes on the body of the person standing in front of the green board get treated as background and vanish.
This is the basic principle of chroma key: using a specific color as the standard and punching a transparent hole wherever that color appears. Put a prepared map or a space scene behind the hole, and you get a perfect composite video.
Why green and blue, of all colors?
There's a clear reason red or yellow backdrops are rarely used. Regardless of ethnicity, human skin and lips get their color from a mix of melanin (brownish), hemoglobin in the blood (reddish), and carotene (yellowish), so they lean toward red, yellow, and brown tones. If you tried to erase a red background, a person's lips or facial skin could be erased and turn transparent along with it.
By contrast, the colors farthest from human skin tones are vivid green and blue. Because they differ most from the human body, it's very easy to cut out just the person cleanly.
In today's digital video, green is used much more often. Most digital camera sensors, modeled on the human eye, have twice as many pixels for sensing green as for red or blue. When you shoot green, the image has less noise (that grainy speckling), and you can separate the color most precisely.
To be more precise: it's not just snipping with scissors
Chroma key isn't a job of quickly cutting out the background like snipping colored paper with scissors. On a real shoot, the computer analyzes the green component of each pixel (the smallest dot on a screen) and calculates an alpha channel (transparency information) from 0% to 100%. That's why even half-see-through parts, like a single strand of hair or thin lace clothing, can be kept semi-transparent.
Another big problem to solve is spill. Light from the bright green backdrop bounces back and often tints an actor's shoulders or hair green. If you leave that light alone, the edges of the shoulders look like they're floating in green once the shot is composited.
So modern video software doesn't just erase the green. It also does a correction that neutralizes the color of the reflected light around the person. Only when careful lighting and digital algorithms work together do you get a movie scene that looks natural.
Common misconceptions
Chroma key must always be shot with a green background (green screen).
Blue (blue screen) works, and technically any specific color will do, even red or yellow. Green and blue are simply the most common because they don't overlap with human skin color and are easy for the sensor to separate.
Where you meet it
A weather forecaster delivers the forecast with weather graphics in front of a huge green wall in the studio.
In a sci-fi movie, actors perform on a green set, and then an imaginary alien planet or spaceship background is added behind them.
A streamer erases the background of their room and broadcasts with just themselves floating over the game screen.
In one sentence
It's a video compositing technique that makes a specific color transparent so the footage blends naturally with another background video.