Blue square, diamond inside, x inside of that.
You can see all the icons at the bottom of the boards page, for example, http://bearinsider.com/forums/forumdisplay.php?f=5
There are also icons to lead you to the first new post in a thread (down arrow) and other features I still haven't explored yet.
Put your mouse over and of the blue icons to see what they do until I can find a good list to post.
Here are some of the icons: