to be in perfect harmony or perfectly in sync, especially referring to a married couple

这对夫妻多年来相亲相爱，琴瑟和鸣，令人羡慕。

the qin and se (ancient zithers) playing together in musical concord

只见台上乐师琴瑟和鸣，清雅的乐声在厅间回荡。