Chroma key.

The Chroma key compositing or chroma keying is a technique that is used for compositing two images or frames together in which a color or a small color range from one image is removed, revealing another image behind it. This technique is also referred to as color keying, colour-separation overlay greenscreen, and bluescreen.This technique is very common for being used for weather forecast broadcasts, to the presenter appears to be standing in front of a large map, but in the studio it is actually a large blue or green background. The meteorologist stands in front of a bluescreen, and then different weather maps are added on those parts in the image where the color is blue. If the meteorologist himself wears blue clothes, his clothes will become replaced with the background video.