Original: In graphics, superimposition is the placement of an image or video on top of an already-existing image or video, usually to add to the overall image effect, but also sometimes to conceal something (such as when a different face is superimposed over the original face in a photograph).
Simplified: In graphics, superimposition is when an image or video on top of an already-existing image or video. This is  usually done to add to the overall image effect or to hide something (such as when a different face is superimposed over the original face in a photograph).