With the rise in living costs, many families have had to tighten their belts.

tighten 是动词，意思是"使变紧、收紧"，来自形容词 tight（紧的）加上动词后缀 -en。 their 是代词 they 的所有格形式，表示"他们的"。 belts 是名词 belt 的复数形式，指"腰带、皮带"。 tighten their belts 是一个英语习语，字面意思是"勒紧腰带"，比喻"节衣缩食、节省开支"的生活状态。