That text just under your screenname (along with the stars beneath it) is your current rank. Your rank changes automatically once you reach a certain amount of posts (for the first hundreds of posts it changes for every 10 posts you make, after that it's for every 20 posts you make).
You can put a custom text beneath your avatar though, such as Sharkalien did up there. You can do that by going to your profile. There you find a text field where you can enter the messgage. Afterwards you can click the "Update Status" button to add that text.