3 Nephi 18:37

and the multitude heard not the words which he spake, therefore they did not bear record; but the disciples bear record that he gave them power to give the Holy Ghost. And I will shew unto you hereafter that this record is true.

The crowd did not hear the words he spoke. So they did not write it down. But the disciples wrote that he gave them power to give the Holy Ghost. I will show you later that this writing is true.

The crowd couldn’t hear what he said, so they couldn’t testify about it. But the disciples testified that he gave them power to give the Holy Ghost. I’ll show you later that this testimony is true.