Spoiler alert

1. When talking about a movie to an audience that has not yet seen the movie, this phrase is used by a speaker to warn that they are about to reveal plot details that may be surprising or unexpected. The thought is that revealing certain plot details will detract from the natural viewing experience, so the warning serves as an opportunity for people to stop listening/reading. 1a. The same idea, but in the context of other media such as books, television shows, etc. 2. Often this phrase is used jokingly following, or immediately proceeding information that is felt to be obvious.