We were relieved to hear you were safe and sound back home.

safe的意思是"安全的"，指没有受到伤害或危险的状态。 sound在这里是形容词，意思是"完好的、健全的"，指身体状况良好。 safe and sound是一个固定搭配的短语，意思是"平安无事、安然无恙"，强调既没有受伤也没有任何损害。