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.
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