An Actress (or sometimes an actor) who plays in a really shitty movie. The movie is a completely flop, or just isn't your type of movie, but she/he is so good looking, that it "makes everything better" and makes the two hours tolerable & maybe even enjoyable.
Occasionally, the movie will survive or even become popular JUST because of this Actress/Actor. (See Examples Below)
Megan Fox is pretty much a walking box office band-aid. The only reason Transformers made it big was because of her! And all her other movies, completely crap without her!