What is they trying to say?

it means to not care much about the things people who don't know you well say about you.
Answer:
basically, if they don't know who you truly are, you shouldn't take any of there stereotypes or whatever personally.
Explanation:
someone can talk all the mess I want about someone at school. but, at the end of the day, they don't actually know who they are outside of school- or really in general tbh.