The Prophet sees in the coming of the Messiah the beginning of an era of liberty and prosperity. There will be general rejoicing, as in the time of harvest, or after a notable victory over an enemy. The same thought is continued in the next verse.
This verse reads somewhat different from the text in the authorized version: "Thou hast multiplied the nation, and not increased the joy." In connection with what follows, this makes poor sense. Nephi had the copy of the prophecy that had been in the possession of Laban, and we may conclude that its text was purer than that of some later manuscripts. The word translated "not" can be a word meaning, "to him;" that is, to the nation, and that makes quite a difference.