What is Green Screen?
Green screen is a special technique for compositing two images or video together. It is commonly used in tv weather broadcasts.
Here’s how it works: The term chromakey comes from the process of isolating the color green and using computer software to make anything that is green invisible or transparent. Since anything green is now see-through, the key allows for another image to show up in those green areas. In the case of weather broadcasts, they choose to put weather graphics in that slot, but any image can be transposed.
Wikipedia gives a more in depth explanation if you would to learn more