The Comments tab in a deal's right-hand sidebar is a simple, chronological comment thread scoped to that one deal.
Type your message and select Post (or use ⌘/Ctrl + Enter). Type @ followed by a name to mention a teammate — pick them from the dropdown that appears, and their name highlights in the posted comment.
You can delete your own comments; you can't edit or delete anyone else's.
Comments are a quick internal note thread, not a full conversation — the deal's Chat tab is the real conversation thread if that's what you need.