He's worried that if he lets his guard down, someone may take advantage of him.

lets 是动词 let 的第三人称单数现在时形式，let 的意思是"允许、让"。 his 是形容词性物主代词，表示"他的"。 guard 是名词，意思是"警戒、守卫"。 down 是副词，表示"向下"的状态。 let one's guard down 是一个固定短语，字面意思是"放下某人的警戒"，引申为"放松警惕"的意思。