What is a white knight
A white knight is a guy who is trying to stick up for a woman no matter how convoluted or wrong the woman is being. This can be on the internet or in person. They do this in hopes of having sex with the woman they are "standing up for".
What is wrong with this behaviour
This behaviour is a bad enabling personality trait that has caused many relationships to fail and be destroyed. It also makes women feel wierd in that their affections must be earned and when they are given things and they don't return those gestures with affection such as kisses or sex then they are demonized as bad women.
It is theorized by many that the origins of this type of behaviour comes from Disney type movies that show incredibley innacurate female to male dynamics in which the male saves the damsel in distress from some thing and then the male gets her affections as a prize or outcome.