Here is an explanation from the main rules.
User Titles
User titles associated with accounts have the following structure:
"new user" -- first two weeks after the registration.
"a regular here" -- at least 20 posts during the two weeks up to date (assigned/removed automatically); does not include posts in the Classifieds section; updated daily at 5:30 AM eastern time.
"community benefactor" -- outlined here.
"top poster" -- a user with the highest number of posts.
"moderator" -- users who help moderate the board.
User accounts registered over two weeks ago (on any given day) but with less than 20 posts in the previous two weeks don't have any title associated with them.