What had Rey understood in The Last Jedi?

Question

In The Last Jedi Yoda said that there is nothing in those books that the girl Rey hasn't understood.

Is there anything specific behind that? What exactly had she understood?

He also said: "We are what they have overgrown." Was it about the force or certain wisdom?

Answer 1

The line is a bit two fold:

"The Aionomica, the Rammahgon, a dozen other mystic-sounding made-up names—the foundation of the ancient faith." ―Luke Skywalker

1. It's referring to the fact that the 8 books Rey takes from the tree library are extremely knowledgeable books in terms of Force knowledge, including information on "Chain World Theorem" (The World Between Worlds), Wayfinders, and the unsolved Thorpe Theorem: a hyperspace plotting conundrum. You can read more about the books, such as the Rammahgon, here, as it was included/explained in The Rise of Skywalker Visual Dictionary.
2. It's also referring to the idea of Yoda's restored sense of faith in the Force. During the course of the original trilogy, Yoda had exiled himself on Dagobah after he felt he greatly failed the Jedi. When Obi-Wan brings young Luke to him, he is initially unimpressed and doesn't really believe that Luke has what it takes to become a Jedi or save the Galaxy. By the time we get to the sequel trilogy and after Yoda has died and returned as a Force Ghost, the role echoes, as the past nearly repeats and Luke has exiled himself on Ahch-to, because he felt he failed the Jedi again. But Yoda's viewpoint since had changed in terms of not loosing faith in both Luke (who he once again prods to do the right thing) and Rey. In other words, he has faith that The Force will guide Rey and that she will be successful in her endeavors. The line about outgrowing what we have learned, is about being willing to 'let go' and trust in the new generation's capacity to learn from the Force beyond whatever anyone before them had tried to teach them.

Answer 2

Wisdom they held, but that library contained nothing that the girl Rey does not already possess.

This is a double entendre by Yoda.

The figurative meaning is that Rey is already wise enough, perhaps due to her time on the island, that she does not need the books anymore. This meaning appears to be what Yoda intends because of the "but", and it makes Luke feel better about the loss of the books.

The literal meaning is that Rey has physically taken the books with her. In the next shot we see that this is indeed the case. This is what Yoda actually means.

It's a joke that only Yoda and the viewer understand.

Answer 3

Yoda: Wisdom they held, but that library contained nothing that the girl Rey does not already possess.

This line by Yoda is a hint that Rey has already taken the Ancient Texts from the temple before it burnt down. We see them aboard the Falcon at the end of the film when Finn is looking through some drawers.

And of course we see Rey has them in her hideout and reads them in The Rise of Skywalker.

Answer 4

In addition to the double entendre mentioned in other answers (that Yoda knew that Rey had literally taken the books with her), and Yoda's belief that Rey would successfully learn to use the Force with or without the books, another interpretation is that Yoda believes that being a Jedi is more about character and intent than any specific scholastic tradition. This would represent a departure from Yoda's earlier characterization of being fairly inflexible and dogmatic towards Jedi tradition (initially refusing to train Anakin due to his age, demanding that Luke continue his training rather than save his friends, etc.), which is why it's shocking to the viewer that he initially appears to be burning the books himself.

The theme of masters burning their sacred texts once enlightenment has been achieved appears in multiple Zen koans, which inspired some aspects of the Force that appear in the other films. For example:

Once there was a well known philosopher and scholar who devoted himself to the study of Zen for many years. On the day that he finally attained enlightenment, he took all of his books out into the yard, and burned them all.

(source)

The Zen master Mu-nan had only one successor. His name was Shoju. After Shoju had completed his study of Zen, Mu-nan called him into his room. "I am getting old," he said, "and as far as I know, Shoju, you are the only one who will carry on this teaching. Here is a book. It has been passed down from master to master for seven generations. I also have added many points according to my understanding. The book is very valuable, and I am giving it to you to represent your successorship."

"If the book is such an important thing, you had better keep it," Shoju replied. "I received your Zen without writing and am satisfied with it as it is."

"I know that," said Mu-nan. "Even so, this work has been carried from master to master for seven generations, so you may keep it as a symbol of having received the teaching. Here."

The two happened to be talking before a brazier. The instant Shoju felt the book in his hands he thrust it into the flaming coals. He had no lust for possessions.

Mu-nan, who never had been angry before, yelled: "What are you doing!"

Shoju shouted back: "What are you saying!"

(source)

In these stories, the lesson is that while tradition and collected teachings may help achieve wisdom, they are not themselves the end goal of wisdom. This seems to be what Yoda is saying, that the practice of doing the right thing is more important than mastery of theory:

Page-turners they were not. Yes, yes, yes. Wisdom they held, but that library contained nothing that the girl Rey does not already possess. Skywalker, still looking to the horizon. Never here, now, hmmm? (pokes Luke with his walking stick) The need in front of your nose. Hmmm?

(Note that this lesson is somewhat undercut by Rey having the actual books, and later using them to fix her lightsaber and track down Exegol in The Rise of Skywalker.)