If you ever wondered what these icons are all about (forgetting that they are the SMF logo [[note to self, change to RAF logo]]) this is the post for you.
The first icon means that there are no new posts in the board. The second icon means that there are. Take for example:
Additionally to telling you if there are new posts in a board, you are able to click on the icon, and you will be shown all the unread posts in that board.