the ebb and flow of something

Từ điển Oxford Advanced Learner 8th

the ˌebb and ˈflow (of sth/sb) idiom
the repeated, often regular, movement from one state to another; the repeated change in level, numbers or amount
•the ebb and flow of the seasons
•She sat in silence enjoying the ebb and flow of conversation.
•He had known her long enough to recognize the ebb and flow of her moods.
Main entry: ↑ebb_idiom