It means: Sincerity is the criterion of God; pursuing sincerity is the criterion of human beings. This sentence comes from "Mencius Li Lou Shang", the original text is excerpted as follows:

There is a way to please relatives: If you are not sincere, you will not be happy with relatives. There is a way to be sincere to one's body: if one does not know what is good, one will not be sincere to one's body. Therefore, being sincere is the way of heaven; being sincere is the way of man. If you are sincere and do not move, you have not yet existed; if you are not sincere, you have not moved.

Translation:

First of all, you must be sincere. If you ask yourself and are not sincere, you will not be able to make your parents happy. There is a way to make oneself sincere. First, one must understand what goodness is. If one does not understand the principle of kindness, one cannot make oneself sincere. Therefore, sincerity is the criterion of God; pursuing sincerity is the criterion of human beings. There has never been a person who was extremely sincere but could not sway others. If you are not sincere, you will never be able to move people.

Extended information:

The last seven chapters of "Mencius" are a collection of Mencius' remarks during the Warring States Period. It records Mencius's debates with other schools of thought and his opinions on his disciples. Contents such as teaching by words and deeds, lobbying the princes, etc. were jointly compiled by Mencius and his disciples (Wan Zhang, etc.).

"Mencius" records Mencius's thoughts on governing the country, political strategies (benevolent government, the distinction between kings and hegemons, people-centeredness, caring about the emperor's wrong intentions, the people are noble and the country is inferior to the king) and political actions. It was written around the middle of the Warring States Period and is a Confucian classic.

The starting point of his theory is the theory of human nature and goodness, and he advocates the rule of virtue. In the Southern Song Dynasty, Zhu Xi combined "Mencius", "The Analects of Confucius", "Great Learning" and "The Doctrine of the Mean" together as the "Four Books". Since the Song, Yuan, Ming and Qing Dynasties, it has been regarded as a book passed down from family to family, just like our textbook.