This is a brilliant summary by commenter "St Michael" at The Punch.

Sticking up for something isn’t bad if you’re defending a valid position. There are other times when sticking up for something—say, eugenics—is indeed a bad thing to do. White Knighting is one of them, because the position a White Knight is defending is not valid.

White Knighting is a pejorative term that really functions as a handy summary for a longer argument. The longer version of the phrase “White Knighting” is to say as follows to the White Knight in question:

“Your contribution on this issue is not out of logical or factual impetus. You are adopting the cause and arguments of the radical feminist movement (which I personally believe is the only feminist movement that is now left; everyone else called it equality and got on with their lives) for one of the following reasons:

(a) you like the coolness of being part of a movement, which massages your self-esteem;

(b) you have a misplaced sense of chivalry that your sisters in the radical feminist movement themselves reject;

(c) you have an inaccurate and somewhat 1950s view of the majority of women as shy, retiring creatures in need of positive discrimination and patriarchal protection rather than equality in that you’re trying to fight their battles for them;

(d) you think you will get laid easier if you do stand up for women on the internet; or

(e) you are jumping in to defend the radical feminist movement without having carefully examined its objectives or arguments, or realised the implications of its extremely sexist point of view.”

White Knighting is a common accusation because the above rationales for men defending the radical feminist movement are, well, common. Or rather, they’re common until White Knights actually sit down and read some of the literature amounting to radical feminist theory and some of the things that are said about men by radical feminists.

Not to mention that most radical feminists, going by their own words on their own forums, are just as contemptuous of White Knights as some men are.