Are you seeing it for all student comments or just some of them? In my experience, those dots mean that I haven't opened/read the item and/or that someone has added a comment to it since my last visit. But I've also noticed that it's not always accurate and will sometimes show items as unread long after I've read all of them.
I don't currently have students whose work I grade, so I'm not sure about the gradebook comments specifically.
