When someone "explains" something to another person about a topic they are already familiar with/understand. This is typically done in a condescending manner under the pretense that the speaker knows more about the topic than who they are speaking to, i.e. they are subordinate

1) Explaining the rules of poker to a poker dealer2)Repeating a point back to someone in your own way, even if they just said the same thing3)Telling a person that you know more about something because of blank, even though they are a professional/have a degree in the field that you do not haveSidenote: it is okay to know a lot about a topic and make this known in conversation, but assuming that you know more than someone else, not accepting if you are wrong , not accepting that they may simply be more knowledgeable, and/or doing so in a condescending manner means that you are mansplaining Note: this definition and examples were built together by both a man and a woman