That's their reputation. You get more boxes, increasing the size of the bar, as you accumulate more rep. See the balance under the green boxes? That's to give a member positive or negative reputation points. use them as you please . I will however say that anything actually worthy of negative reputation, trolling, flaming, excessive curses etc, should probably be reported to the mods with the ! button next to the balance.
Not really. It's mostly there to let members know when their efforts are appreciated by others. Though, you can float it like some spiffy cyber bling if you want .