It means: it is wise to know people. You know others, you are wise, you can understand others only by knowing yourself, and you can use others only by knowing others.

This sentence comes from Laozi's Tao Te Ching.

Wisdom is the wisdom of self. Ming is the Ming of the mind. "Knowing people" knows the outside world; "Self-knowledge" is clear in Tao. A wise man knows people but not friends, and knows the outside but not the inside; Smart people know themselves, both inside and outside. Wisdom is conscious, formed in the day after tomorrow, comes from the outside world, is the knowledge and understanding of superficial phenomena, and is limited and subjective; Ming is an understanding of the nature of the world, which is infinite, objective and comprehensive. If you want insight, you must return to the Tao. Only those who know themselves are truly enlightened.

He who can understand others is wise, and he who can understand himself is brilliant. He who can defeat others is powerful, and he who can defeat himself is truly powerful. Those who know that they are satisfied without delusion are rich, and those who are diligent are ambitious. People who are consistent in words and deeds can live long. Those who live in the world with their bodies dead and their spirits alive are the ones who really live long.