Mansplaining isn’t just the act of explaining while male, of course; many men manage to explain things every day without in the least insulting their listeners.
Mansplaining is when a dude tells you, a woman, how to do something you already know how to do, or how you are wrong about something you are actually right about, or miscellaneous and inaccurate “facts” about something you know a hell of a lot more about than he does.
Bonus points if he is explaining how you are wrong about something being sexist!
Think about the men you know. Do any of them display that delightful mixture of privilege and ignorance that leads to condescending, inaccurate explanations, delivered with the rock-solid conviction of rightness and that slimy certainty that of course he is right, because he is the man in this conversation?
That dude is a mansplainer."
Also extends to anything that begins with “Well, as a MAN…”