Little ipod things below your name

I haven been able to find this with search, so please dont flame me, but what do those little ipod icons undeneath your name mean/represent. Some people have them filled, but others dont.

A new iPod filled up means you got to a certain number of posts.

thats what I figured, but i wasnt sure. How many posts do you have to have for each one?

Not sure exactly.

ok thanks anyway though