你知我长短x我知你深浅x
[nĭ zhī wŏ zhăng duăn x wŏ zhī nĭ shēn qiăn x]
Roughly translated as 'you know my ups and downs; I know your ins and outs'. It refers to mutual understanding between two people, emphasizing familiarity and depth of knowledge about each other's strengths, weaknesses, moods, etc., signifying a close and deeply connected relationship.