Green Screen (can also be blue) is a backdrop that allows fora change in background in editing. Herewith the wiki definition:
Chroma key compositing, or chroma keying, is a visual effects / post-production technique for compositing (layering) two images or video streams together based on color hues (chroma range). The technique has been used heavily in many fields to remove a background from the subject of a photo or video – particularly the newscasting, motion picture and videogame industries. A color range in the foreground footage is made transparent, allowing separately filmed background footage or a static image to be inserted into the scene. The chroma keying technique is commonly used in video production and post-production.