She advised him not to let his guard down during the negotiations.

let 是动词，意思是"允许、让"。 his 是代词 he 的所有格形式，意思是"他的"。 guard 是名词，意思是"警戒、防卫"。 down 是副词，意思是"向下、降低"。 let one's guard down 是一个固定短语，字面意思是"放下防卫"，引申为"放松警惕"。