Dante W. answered 08/04/19
Film Director, Poet, Screenwriter, and Author. I love creativity!
It generally means that they have an unscripted appearance on set or that they are not credited in the movie. For instance, in Thor: Ragnarok, Matt Damon is seen briefly performing in a play depicting the events of an earlier movie and is not credited in the end credits for his role. This is a cameo.