Deep thoughts, Idek why I did this ♀
Write what you know.
The above statement doesn't mean write what u literally know. In my opinion it means we write our thoughts into words, we write our feelings our observations about the world. When a person who is a writer is angry/mad instead of punching something or taking it out on something else they seem to write it down into a deep statement one that helps them understand what happened, or when they're sad writing would help them, they'd write an angsty statement/ paragraph whatever. I think that's what the sentence "Write what you know" means.