If you tighten your belt you try to spend less money because you haven't got as much to spend as you did before.
Since I started working part-time I've had to tighten my belt.
Celebrating Christmas is going to cost us a lot of money so we're all going to have to tighten our belts.
Being tight is used to describe people who do not like to spend their money.
My brother is so tight; all he bought me for my birthday was a cheap pair of socks!
In judo or karate a coloured belt is worn to show a level of expertise. A black belt is worn by a person with the most experience.