What is the "postbit"?

[TheHitman]

I feel like an idiot asking this, but what is the postbit template and how do I get around to editing it? I am looking to add a new profile field which will show up under someone's avatar. I have the necessary steps to do this, but the instructions say "open up the postbit template and do this..." and I'm left asking myself, "Okay, how do I GET to the postbit template?"

I did a forum search already, but no new information was to be found. Everyone seems to know where to go to get it.

Any help is appreciated.