This term is used to talk about two different techniques. The common thread is that they both map time to space.
The traditional slitscan effect was created by running a piece of film in front of a slit. Anything moving in front of the slit will be 'scanned' in according to the time it went in front of the slit. It was often used to determine who won a race.
A more modern interpretation of this effect is to displace strips of a video across time. For example, the left hand side of the video might be 4 seconds ahead of the right hand side of the video.
I've enjoyed using this technique to take videos of flowers in the park. There's a dissonance between the natural setting and the aesthetic of flatbed scanning.