All threads have a read/unread status. Unread show as slightly bolder in appearance. Once you read it...
But then you can mark all threads in a forum as read. But that doesn't do anything significant by itself, until you use filters: Think "active topics" except "Only show unread" (across all forums). What would be really nice, but too complicated from a UI pov, is forum lists that are "only unread". It can be done using the filter, but given I've got the filter set to a default of "all unread", it would get too fiddly.