It means to praise and praise others' good deeds in public, and to admonish others for their mistakes in private. It is the original sentence in Zeng Guofan's family letter.

Everyone’s work, even if it is wrong, is someone else’s idea and hard work, and it must be valuable to him. Directly pointing out other people's mistakes without any reservation or twist is tantamount to directly denying other people's efforts. Therefore, the best way to point out a person's mistakes is to first affirm his behavior and thinking, and then tell him that your actions may not be particularly effective. It is better to change your thinking. Also, try to use less words like "wrong" and more words like "I think this is better."

Another point is that when pointing out other people's mistakes, you must have a result-oriented thinking.

The so-called result-oriented thinking is to tell your friends how much damage your mistakes will cause if you follow your approach. Telling the other party the possible consequences will make the other party seriously consider your suggestions and reflect on their own mistakes.