After hours of intense study, we went outside to run around and let off steam.

let 是动词，意思是"允许、让"。 off 是副词，表示"离开、脱离"的状态。 steam 本义是"蒸汽"，在这里比喻为"压力、紧张情绪"。 let off steam 是一个固定短语，字面意思是"释放蒸汽"，比喻为"发泄情绪、释放压力"。