1) At least two named female characters who
2) talk to each other about
3) something other than a man
It's meant to point out how women aren't really seen a central points to the action, and even in scenes between two or more women the object of their discussion is usually the male character at the center of the film.
Movies that "fail" the Bechdel test are usually said to do so because male executives don't believe that movie-going audiences would watch a movie about a woman/women, despite recent evidence to the contrary (Juno, Terminator w/ Sarah Connor, etc).