He's worried that if he lets his guard down, his staff will start to become lazy.

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，指代"他的"。 guard 是名词，意思是"警戒、防卫"。 down 在这里是副词，表示"向下、降低"的状态。 let one's guard down 是一个固定短语，字面意思是"放下警戒"，引申为"放松警惕"的意思。