Messages and message requests
Who can message you, and how requests work.
Messages are private between the two of you. If someone you don't keep up with messages you, it arrives in Requests. You can reply to accept it, or remove it. They cannot see whether you have read it.
Choose who can message you in Settings > Privacy & safety: anyone, only people you keep up with, or nobody new. Moderators only see a message if one of you reports it.