The greatest movie in the history of the world. If you are Asian, from New Jersey, Jewish, or smoke weed, you will definitely love this movie. But even if you don't, you will still love it, because it is the funniest movie of all time.
Cool Person: Guess what?
Loser: What?
Cool Person: I watched Harold and Kumar Go To White Castle twice in the past 24 hours. And then I went to white castle.
Loser: Oh.
In forum parlance, it means rushing to the aid of another poster for whatever reason. An accusation typically reserved for when a poster is being hammered by another poster(s) and the accused leaps in to defend them, usually without thinking things through as to why the person is being attacked.
Poster A: <Says somethingunfunny>
Poster B: Shut up, douche.
White Knight: Hey, leave him alone. Why are you all ganging up on him?
Poster B: Quite white knighting, douche.