1 Nephi 14:23

wherefore, the things which he shall write, are just and true; and behold, they are written in the book which thou beheld proceeding out of the mouth of the Jew; and at the time they proceeded out of the mouth of the Jew, or, at the time the book proceeded out of the mouth of the Jew, the things which were written, were plain and pure, and most precious, and easy to the understanding of all men.

So the things he will write are right and true. They are written in the book you saw coming from the Jewish person. When they came from the Jewish person, the things that were written were clear and pure and very precious and easy for all people to understand.

“Everything he writes will be right and true. Look, it’s all written in that book you saw coming from the Jew’s mouth. When it first came from the Jew’s mouth—when the book first appeared—everything written in it was clear, pure, extremely valuable, and easy for everyone to understand.”