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 something unfunny>
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.
White Knighting typically occurs when their are two men and one girl. White Knighting is easy to spot and very annoying when you are trying to get laid with a crappy wingman.
White Knighting is when one man is trying to get a girl into his bed while in the same area as his friend, his friend (if he is a White Knight) will typically attempt to disperse the conversation by presenting his chivalrous words such as: "Have some respect for women, Johnny" or "Miss, he's just trying to get laid" while simultaneously attempting to get laid himself by showing respect to the women. A White Knight will never act chivalrous when he is not around another guy.
Jim: Hey babe, you wanna go back to my place? I'll make you some food.
Lancelot: Jim, your just trying to sleep with her, I see this every time we go out
Girl: Excuse me? I'm not a slut. (leaves)
Jim: What the hell Lancelot, quit your White Knighting, I was about to get laid.
Lancelot: Dude I know! We should totally go out again tomorrow! I'm about to get that girls number, she's totally into me.
When a male, preferably after sex, stands before a kneeling female, whom he preferably just had sex with, and cums on both of her shoulders before declaring her a "knight of the white table." The more cum used, the better.
Male: "Yeah, I brought her back, fucked her, then white knighted her."
Friend: "Ah, always great to see a new addition to the white table. White knighting for the win!"