When the circle or the star are filled, it means you have not read all the posts in that thread. Instead of clicking on the title of the thread, you have to click on the circle or the star. You have to be inside a forum to be able to do this, you cannot do it fro the home page because you have no circles and stars there.
Nop, none of your suggestions seems to explain the circles and stars. The fill means you have not read the last post in that thread, for both stars and circles. I am not following anyone, and I have currently no members blocked.