Is this a literal expresion or does it is an idiomatic expresion? if the later, what does it mean?
It can also refer to experience. "Once you have a few more lessons under your belt, you will be able to play the piano better."
belt - Definition from Longman English Dictionary Online
<below the belt>
unfair or cruel:
- That was a bit below the belt, Paul.
- The comments hit below the belt (=they were unfair or cruel).
<have/get something under your belt>
to have achieved something useful or important:
- a secretary with several years' experience under her belt
Which are you mentioning?