Idiom: to tighten one's belt (used as a verb)
Tim: Do you want to go to lunch?
Sally: I brought lunch, but thanks for the invite.
Tim: Wow! You never bring lunch!
Sally: Well, Justin lost his job, so we're gonna have to tighten our belts until he finds something new.
Tim: That's too bad. I hope he finds a new job soon.
Meaning: The expression "to tighten one's belt" means to spend less money. The idiom is most often used when the person is spending less because he or she now has less many, as in the example. Here, Sally explains that her husband Justin lost her job, so she can't go out for lunch.
Father: Kids, we've decided to buy a new house.
Son 1: Awesome! Will I get my own room?
Son 2: I want my own room too!
Mother: Yes, you'll both get your own rooms.
Father: But, the mortgage on the new house is more expensive, so we're going to have to tighten our belts.
Mother: That means no pizza or going to the movies for a while.
Son 1: That's OK! I can't wait to see my new room!
Meaning: In the second example, the father uses the example to explain that the family will have less money because of a larger mortgage payment (so they are making the same amount of money but have more expenses now). Notice that the idiom is commonly used with the modal "have to" (seen in both examples).