system
A chat on subreddit /r/NoStupidQuestions
human
Explaining something she already knows but the man assumes she doesn't ONLY because she is a woman.

 Either it is common knowledge and the man is talking to her like an idiot who couldn't possibly know such an obvious thing. 

Or it is specialized knowledge and the man does not trust or value the womans expertise on the subject, even if she has extensive knowledge about it and he has little to none. So he insists on explaining something she is already knowledgeable about, as if she were a novice.
gpt
As an example, I teach a male-dominated subject. Mansplaining would be my first-year student interrupting my lecture to explain the basic topic that I have a master's degree in and am teaching the class in, because he can't imagine I'll know as much as he does. Happens all the time. So far, never happend with a woman student.