A chroma-key workflow records or renders a subject against a highly distinct solid color, commonly green or blue. Editing software identifies pixels near that color and turns them transparent while trying to preserve the subject.
The technique is fast and widely supported, but it is an approximation. Semi-transparent pixels, motion blur, shadows, reflections and colors similar to the key can create rough edges or remove parts of the intended image.
ELI5
Chroma key removes a chosen background color from a video. It is the technique behind a green screen, where the editor makes the green transparent and places the subject over something else.
For example, an animation can be exported on a bright green background and keyed over a website. The method works quickly, but soft glows and transparent edges may include mixed green pixels that are difficult to remove cleanly.
