你来我往 (nǐ lái wǒ wǎng)。
Initialy,the meaning was a big crowd of people coming and going.Like in a crowdy street or a big market etc. But over the years this expression lost its initial meaning and now is used in another meaning.
This expression is used to describe any action between two parties (people,institutions,companies,animals etc),
which one party does for the other and then the other returns it (the same action) to him.
* when friends meet in a restaurant,one invites the other and next time the other will invite him.
* In making language exchange, there is mutual help.One time one teaches the other his language and the other time the other will tech him his own.
* Also in the negative sense,one shouts at the other and then the other shouts back at him. also,one beats the other and then the other beats him back.
Example of a sentece:
来跟朋友相帮合作需要懂你来我往 ( lái gēn péng yǒu xiàng bāng hé zuò xū yào dǒng nǐ lái wǒ wǎng)
in order to cooperate with friends one must know Reciprocity.
人来人往 (rén lái rén wǎng)
Nowadays this is the expression to describe a crowd of people coming and going.
节日的前一天，市场里很热闹，人来人往。(jiē rì de qián yī tiān ，shì chǎng lǐ hěn rè nào ，rén lái rén wǎng) 。
The day before the festival, the market is very lively and bustling