It's a rough metric of the behavior and posting content of a user, and the content of the forum as a whole. Users which post threads and posts get paid 'bits' for them. Users which must be moderated all the time will quickly end up with negative bits. And so forth.
These bits can be spent in various ways. Most usefully, you can pay bits to create a thread in the emergency forum. You can also pay bits to set a custom user title. And so forth.