The superficial meaning of this sentence is, what is the law? It's "middle". Emperor Qianlong was an emperor who learned Confucianism extremely well. The word "中" is the word "中" in "The Doctrine of the Mean". The so-called impartiality is called neutrality. What are the laws of heaven and earth? Confucius said before that it is benevolence, but Laozi said that it cannot be said. If it is said, it is not Tao. Qianlong believed that Tao (the law of all things) is a Chinese character. This Chinese character means between the two ends. That is, everything needs to be just right and not go to extremes. Maybe this is its original meaning. If you want to know more about its meaning, I suggest you read "The Doctrine of the Mean" and "The Analects of Confucius" carefully. Maybe you can find better answers in them.