What is a sphere reflection capture?

Can somebody explain to me what is a sphere reflection capture in layman’s terms?

How does it work and when it is used for what?

You put it in your level and it saves an image of its surroundings. Then objects that have a reflective surface can use that image in their reflection, because it would be too slow to capture the scene for every reflective object, especially if they move and have to be updated every frame.