At least two women
Who talk to each other
About something other than a man.
Once you start thinking about it, it's really a revelation what a simple, objective and accurate measure this is for the importance of women in a given story. It's true, a lot of “chick flicks” have women talking about men—but they pass, because they also talk about other things. It's also true that an action movie can have a strong female character who kicks ass, is treated as an equal and doesn't end up in a refrigerator; but if that is the only female character, or if her role is so peripheral to a male that she does not talk to the other women about anything else, then that film fails the Bechdel Test, because it is primarily about its male characters. What really starts to bake your noddle, as the Oracle said, is to realize that most films fail.
I don't think it's a perfect measure. Sin City actually passes, and I find that a terribly sexist movie. All of the women portrayed are sex workers of some sort—all of them—except for one social worker who spends the entire movie naked and loses her hand. Let's also not forget the gang of prostitutes who have enough firepower to knock over an armored car, but would rather continue making a living by selling themselves. It's not that I think prostitutes don't deserve respect—just that black-market prostitution, as it occurs in the real world, does not; and that to have such a narrow range of characters in a story is more a product of infantile male fantasy than of genre or setting. On the other end of the spectrum, I was relieved that the Matrix trilogy passed, because I couldn't recall any specific instance where multiple women discussed anything other than Neo. (It's worth noting here that presumably, the conversation in question need not be private.)
A movie that fails the Bechdel Test is not automatically anti-woman...it's just not pro-woman. More specifically, it's not as balanced and complete a picture of the human experience as a movie that does. I'm guessing that most of the movies that pass, except for those rare films with an all-female cast, have at least one brief conversation between two men about something other than women.
A good movie is one that tells a human story, and almost half of all humans are male. A movie that fails the Bechdel Test may still be a good movie. But until there is parity in whose story gets told and how, the Bechdel Test provides a useful and fun gauge for the health of our popular culture and the movie industry.
(Bend it like Bechdel! If you want to see how your favorite films stack up, or to chime in on one that's been overlooked, click here. The page includes a lof of other cool links!)