It means: if you have questions, if you don't ask the teacher, those who become difficult problems will eventually fail to understand. This is also a mystery: his doubts.

1, source

This sentence comes from an argumentative essay in Han Yu's Shi Shuo:

Ancient scholars must have teachers. Teacher, so preach and teach to solve doubts. People are not born knowing, who can have no doubt? If you are confused and don't follow the teacher, you will be confused and you will never understand.

It means that people who studied in ancient times must have teachers. Teachers can rely on imparting truth and answering doubts. People are not born to understand reason, who can have no doubts? If you have questions, don't ask the teacher for advice. Those who become difficult problems will not understand in the end.

Step 2 introduce

Shi Shuo is selected from the anthology of Han Changli. This article was written by Han Yu to his student Li Pan when he was a doctor in four universities. This paper criticizes the wrong idea that "scholar-officials" were ashamed to learn from teachers at that time, and advocates learning from teachers. At the same time, it is also a public reply and solemn refutation to those detractors. In the literati class at that time, there was a general psychology of respecting "family law" and neglecting teachers. It can be seen that it is commendable for Han Yu to write Teacher's Theory and publicize his views with great fanfare.