Superimposing describes the process of overlaying and combining multiple images. Video clips are completely opaque by default, but superimposing them requires transparency. When you make clips on upper video tracks transparent, they reveal clips on the tracks below.
In Premiere Elements, you can quickly and easily make entire clips transparent by using the Opacity effect. In addition, you can apply any combination of opacity, masks, mattes, and keying to modify a file’s alpha channel, which defines the transparent areas in a clip. More advanced keying effects let you make specific colors or shapes transparent.