I guess you mean this:
= No new topics
= No new topics (popular forum)
= No new topics in a forum you've previously posted in
= No new topics in a forum you've previously posted in (popular forum)
= Locked topic
= Moved topic
If an icon is red, it means there is an (are) unread topic(s) or post(s) in that forum or topic; e.g.
= Unread topic(s)
= Unread topic(s) in a popular forum
These icons will change in the near future, but for now this is how it works...