A term used when a person (usually a man) explains something to someone (usually a woman) in a way that is inherently condescending.
Often misunderstood as any time a man disagrees with a woman - thereby deeming it a sexist slur against men as it implies that any difference of opinion with a woman automatically makes him a sexist - it is about the context of what is being explained.
Thus, while whatever it is usually IS factual, it is insulting in its delivery.
Example: Two coworkers (one man, one woman) work in finance. They go to lunch. When the check comes, the male coworker slyly offers to teach her a "trick," then proceeds to explain how to calculate a 10% tip.
The female coworker might say, "He mansplained how to move a decimal."