In the Northern and Southern Dynasties (nán běi cháo 南北朝,420-589AD) when the State of Chen (chén guó 陈国) was facing its demise, Xu Deyan (xú dé yán 徐德言), husband of the princess, broke a bronze mirror into halves. Each of them kept a half as tokens in case they were separated. Soon afterwards, they did lose touch with each other, but the two halves of the mirror enabled them to be reunited.

This idiom is used to refer to the reunion of a couple after they lose touch or break up.

南朝的陈国将要灭亡的时候，驸马徐德言把一面铜镜破开，跟妻子各留下一半。双方约定：如果将来夫妻失散了，就把它当作信物。后来，夫妻二人真的失散了，凭借着各人留下的半面镜子，他们最终又得到团圆。

“破镜重圆”这个成语比喻夫妻失散或分离后重新团聚。

破镜重圆 (pò jìng chóng yuán)
【翻译】A Broken Mirror Made Whole Again.
【释义】比喻夫妻失散或离婚后重新团聚。
【例句】王先生和她妻子离婚多年了，前不久破镜重圆，重新踏入了婚姻的殿堂。
【近义词】和好如初、言归于好
【反义词】覆水难收、一去不返