The act of someone from outside a religion trying to explain aspects of that religion to someone who practices it. Note: It isn't always derogatory not everyone reads their own holy book and not every outsider is ill informed, but it does often come off as annoying to the person receiving the explanation.
Buddhist: "So yeah my religion is Buddhism"
Atheist: "Akshually Buddhism isn't a religion"
*The Buddhist in a full Kasaya and shaven bald*
Buddhist: "Are you literally Faithsplaining to a monk?"