To tighten your belt means to spend less money than you did before because you have less money.

For example:

I’ve had to tighten my belt since I stopped working full-time.

Clearly, if you are spending more than your income, you’ll need to tighten your belt.

Businesses were tightening their belts and laying off employees.

 Most people have to tighten their belts a little when they retire. 

Tradução: apertar o cinto por não poder arcar com mais despesas.