A person whose attendance at a social gathering guarantees your own attendance. This may be a crush, significant other, or close friend. Upon being told of their attendance, you immediately decide you will go.
Taken from the 1999 film "The Matrix" in which the main character is told by his computer to "follow the white rabbit" and shortly after accepts an invitation to go clubbing with his friends only after he sees that one of them has a white rabbit tattoo. This in itself is a reference to Lewis Carroll's "Alice's Adventures in Wonderland".
Guy 1: Hey, you going to Jim's party tonight?
Guy 2: Nah man, I'm gonna stay in.
Guy 1: Aw, c'mon, everyone's going. Daniel, Tom, Tanya-
Guy 2: Well. . . yeah, ok, I'll go.
Guy 1: You totally only said that 'cause Tanya's going!
Guy 2: What can I say, dude? She's my white rabbit.