蓦然回首你已远走
[mò rán huí shŏu nĭ yĭ yuăn zŏu]
It translates to 'When I suddenly look back, you have already gone far away.' This line expresses a feeling of sudden realization or regret upon discovering someone important is no longer there or has moved on with life without the individual speaking. It comes from a classic poem, expressing sorrow for someone being away.