Basically on a forum, you can read through the topics people post and if you're interested and you want to participate in the discussion then go ahead and reply to them. Or if you have an idea for a discussion, or you have a question, or find a news article, etc., you can start a new topic yourself to start a discussion.
Explanation of the stars:
http://www.ahsforum.org/forum/showth...&threadid=4145