does anyone know what the green bar on top of the "joined" part mean?
It is a (mostly useless now) indicator of how many "thanks" you've received. For example, if you post an answer to someone's question, they might appreciate the response and click the "Thanks" button (at the bottom right of your post). The more thanks you receive you'll see more bars. I don't get the impression many people pay it any mind anymore, though I use it myself. Alot. I think it's only right to thank someone for their efforts.