The quote below is from the online publication Videomaker and it explains Green-screening very well. We can only add this: WHAT A WAY TO EARN A LIVING! CGI artists must be some of the luckiest people in the world.
“Through the magic of video effects and technology, you can superimpose your subjects onto virtual backgrounds, place them over animated digital backdrops or transport them to a desert oasis. You can shrink down a full-grown man so he can stand on a table top, use visual effects to make him fly through the sky like a superhero or simply simulate your own TV weather report. But to do it right, you’re going to need a lot of green. No, we’re not talking about money. The green we mean comes in the form of a green screen. The secret to pulling your subject out of the real world and placing him or her into a digital domain is chromakey, and that means going green.” [vm_ads:segment_break:1] (https://bit.ly/2a4vDGq – accessed 29.01.2019)