The line under your username is called your user title (or "custom title" if you're typing in your own title instead of using what the site generates for you). The site
does give everyone default titles depending on what usergroup you belong to and how many posts you have, but most people create custom titles.
To create/change your custom title,
hover your cursor over your username on that bar at the top of the screen, then click on "personal details". One of the first few lines should contain a box that says "custom user title", where you can type in a new title for yourself.