Green screen (also blue screen) is the use of a single color as a backdrop in filming to make it simpler and easier to add backgrounds, characters or other images.
The pure, uniform green or blue color is easily taken out of the resulting visual content with automatic software color selection and removal tools and the desired background inserted in its place. Green and blue screens are most commonly used, although any color could work in the same way. Bright green is often chosen because it's not a color that people wear very often.
One familiar use of green screens is to add maps to the background in weather reporting, behind meteorologists. Green screens are often used in movies to present created environments or simulate real locations. In films depicting far away or imaginary places, compositing may be used along with green screens to create convincing background visuals. Compositing is the combination of multiple layers of images or video elements to render a final still or moving image.
CGI (computer graphics imagery) effects are added to films with compositing as well. Adding the 3D rendered image files for backgrounds is accomplished with green screens. The addition of rendered objects or characters to filmed scenes does not require a green screen, however, as the frames are rendered with transparency information included.
The only caveat of green screening is a warning that subjects should not wear the background color or those parts of their bodies will vanish on screen.
The use of green screens in movies and how it works: