Definition
Used to indicate that something, such as a rule, is commonly known and accepted by everyone.
心照不宣
In the office, it is understood that cell phones should be turned off during meetings.
在办公室里，大家都知道开会时应该关掉手机。
In public places, it is understood that one should maintain quiet.
在公共场所，大家都知道应该保持安静。
It is understood that if you borrow something, you should return it.
大家都知道，如果你借了东西，就应该归还。