It is an idiom, an aphorism and a famous saying. From "The Analects of Confucius", it means "Don't do things to others that you don't want to do." Many idioms come from ancient classics and famous sayings. For example, from the Analects of Confucius, there are "Similar to the wise, let bygones be bygones, not ashamed to ask subordinates, draw inferences from one instance", "We are all brothers in the four seas, how can we kill a chicken with a bull's knife" and so on.