Kat Tanaka Okopnik says:
Mansplaining doesn’t mean “explaining done by a man”, it means “a man chose to barge in with explanations without checking the credentials of anyone else in the conversation, assuming his were better than anyone else’s in the room — i.e. that he was the expert by default”.
It is the consequence of a culture that devalues non-men, especially non-white non-men. The individual man who does this is just as likely to be unaware that he’s doing this as he is to be a blatant sexist. It’s only avoided by conscious consideration of context and a willingness to cede the pedestal to others.
I’d just like to point out that while this behaviour is no doubt much more common in men, it’s also been observed in women. By me.