it is a feature on some cameras where the films is exposed to the light (e.g. when you take a picture) twice, so the result is two over lapped feature. I used this a bit as a kid to try and convince my friends that i'd seen a ghost by taking a picture of a darkish corridor in my house then (without moving the camera at all) dressing up scary like and taking a picture of myself walking down the corridor. The result with the double exposure was a see through image a me walking down the corridor ... woooo scary