I would guess they are a way of marking messages you have read so that next time you log in you know which you have seen before and those you have not. So for example you may have 1000 messages but only 3 of them are new therefore you wouldnt want to read all 1000 each time you log on. Most boards have an auto read feature meaning once you have viewed them they become 'read' the unread feature is then used to highlight those you may wish to read in the future