On an other forum I'm suscribed to, that's the admin role to set them.
Once you reach some "level" of activity, mods give you some (really) small text to put there. For example, mine is "Wonder ?" (in reference to wonderbras). So it's just a funny thing, and the mods can control it (it's way funnier when you don't choose it ^^).

Sometimes, it's used to give status to members, on help forums and such, so that you can quickly know if you can trust what a user says (most of the time, it's related to the number of posts)