Chroma key is an editing technique in which two images are composited together to create a single, finished image. This editing technique is sometimes referred to as blue or green screen, referencing the Backdrop used to film the subject of an image which will be subjected to the chroma key process. This editing technique is used extensively in film and television to create a range of special effects, and the basic concept is familiar to many film fans as a result.

Basicly what that means I shoot the subject using a green or blue backdrop and during
editing I remove the green or blue backdrop, leaving a clean image to superimpose on any backdrop of your choosing as shown in the samples below.