I think you are asking about the Reputation system. As members give you Rep points for threads and post that you make that they think are deserving, you will get more Rep points (can be seen in your Profile page on the Reputation tab). The more points you get, the more of those green bars will show up next to your name... and when reach certain levels, site medals will be issued.
For more info on it you can look at the Reputation entry on this page
UK Wing Commander