A white knight describes the good guy in the story, the one who rescues the distressed damsel.
By extension, and applying just a hint of sarcasm, a white knight has become that guy who is really sweet and lovely to women, performing small modern day heroic services - walking her back to her car, offering to rescue her from overbearing dates, holding the door - because that is his strategy for getting laid.
The subtext here is that he is also quite likely to be the man who complains when his clever strategy fails to get him laid. You know that guy who whinges about being friendzoned as though his chivalry means he is entitled to at least a pity fuck? Yep, that one.
"He's a real white knight," she said, rolling her eyes. "He never offers to walk the older women back to their cars, does he?"
In the internet age, there is another, even darker angle. Sometimes a white knight is a man who publicly proclaims his feminist views, shares your feminist posts and likes to join in discussions about how angry it makes him when men take advantage of women - but offline, he routinely intimidates his domestic partner, blames her for 'making' him act violently, or fails to do his basic parenting duties. The 'white knight' angle online or in public is a way for him to convince himself that he is a good guy, really, after all.