To fund the new infrastructure, taxpayers were asked to tighten their belts.

tighten 是动词，意思是"收紧、使变紧"，这里引申为"缩减开支"。 their 是代词，指代前文提到的"纳税人"的所有格形式。 belts 是名词 belt 的复数形式，本意是"腰带"，这里用 tighten their belts 这个习语表示"勒紧裤腰带"，即"节衣缩食"的意思。