It has to do with the number of posts you have made and whether someone has donated rep power to you (the scales in the blue bar). It's a blury gray area that is hard to explain. Right now it doesn't really do much but give people something to ask about. Maybe down the road it will be used for a site promotion. Basically it shows your reputation within the SMF community.. kind of.
Thanks PigCycles, that makes it less cloudy!