It's when a person without proper knowledge chips in unsolicited and starts explaining something to another person who is better in the subject (might even be an expert in the field). The name comes from men usually doing this to women ("Well actually...") but evolved ever since
Discussion
So when someone asks a question and someone else responses then this doesn't count. It's tricky however on the Internet because the semi-open nature of the discourse here muddies the water and someone joining a discussion in good faith can end up labelled as mansplainer but that's life and there will always be dangers