It means: all reading should be familiar with it first, so that everything in it seems to come from my mouth; Excerpted from Zhu's Rules of Training and Learning, the original excerpt is as follows:

Generally speaking, you must be familiar with this book before reading it, so that everything you say comes from my mouth. After careful consideration, make its meaning come from the heart, and then you can get it. As for the doubts about the meaning of the text, if you have different opinions, you should also consider it with an open mind and don't hesitate to choose.

Translation:

All reading must be familiar with it first, so that everything in it seems to come from my mouth. Think about it further and make its meaning seem to come from what I think in my heart, then I can learn something. As for those who have doubts about the meaning of the article, everyone's comments are confusing, so you should calm down and think carefully, and don't rush to choose.

Extended data:

The ancients said, "When a book is read many times, its meaning naturally emerges." In other words, if you read the book well, you will naturally understand its meaning without relying on others to explain it.

The ancients once said: reading pays attention to "three", that is, when reading, you should concentrate, read carefully and read aloud. If the heart is not in the book, then the eyes will not read it carefully. Since the heart and eyes are not absorbed wholeheartedly, they just look at it casually, which is definitely unforgettable, even if they remember it, it will not last long. Of the three, the heart is the most important. The heart is here, and the eyes are here.