A walk-on in a movie or television show is a very small role, similar to a cameo. However, cameos are reserved for the very famous, while any Joe Schmoe can do a walk-on.
Walk-ons are different than extras in that extras don't talk and exist to fill in the background of a scene. A walk-on actor's appearance is usually important to the plot, although usually very short - usually under a minute.
If you get a walk-on role in a horror film, you will probably be killed very quickly and unexpectedly - in the movie, that is, not real life.