On the surface, this sentence means, what is law? Is to win. Emperor Qianlong is an emperor who knows Confucianism very well, and the word "zhong" is the word "zhong" in the golden mean. The so-called impartiality is right. What is the law of everything in heaven and earth? Before Confucius said it was benevolence, Lao Tzu said it was unspeakable, and it was not Tao to say it. According to Qianlong, Tao (the law of all things) is a Chinese character. This Chinese character refers to both ends. In other words, everything needs to be just right, not extreme. 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 carefully. Maybe you can find a better answer in it.