It's a post count thing that theoretically indicates the experience level of posters here... here's a thread that details the levels. I found this by going to the forum where I thought I would most likely find the information (in this case 'community'), hitting 'search' from the menu bar at the top of the page, and entering keywords 'membership level'. Voila!
http://www.flyertalk.com/forum/Forum95/HTML/000798.html