She had been thinking about moving abroad for ages, and last month she finally took the plunge.

took 是动词 take 的过去式，take 的基本意思是"拿、取"，在这里表示"采取（行动）"。 the 是定冠词，表示特指某个事物或情况。 plunge 的本义是"（突然）投入、跳入"，常用来比喻"突然采取重大行动"。 took the plunge 是一个固定短语，字面意思是"跳入水中"，比喻"下定决心做某事（尤指重大决定）"。