闺蜜比海还深闺蜜比天还蓝
[guī mì bĭ hăi hái shēn guī mì bĭ tiān hái lán]
It describes the bond between best female friends ('闺' - inner chambers used metaphorically for private bonds; '蜜' – honey, indicating affectionate closeness) being deeper than oceans and wider than skies, expressing deep and unbounded loyalty amongst friends.