Double or multiple exposure-
1)exposing the film more than once without advancing the film. Some cameras have this as a built in feature, some need to be tricked to shoot the second time without advancing the film.
2) opening the shutter (camera on tripod) in a darkened room and using a spot or flashlight to “paint with light” on your subject.
3) Run the film through the camera twice. Mark how your film lines up when you load your camera, shoot first exposures on the roll and then rewind the film but not all the way. Reload the same film in the camera and make 2nd exposures on top of the first.