The new guide is an angel from the heavens. Angels speak divine messages to humans, and the presence of the angel sanctifies this important message. The vision had been of a virgin, and now the explanation comes as to why she is important. The message is: “the virgin whom thou seest is the mother of the Son of God, after the manner of the flesh.” In the original manuscript, and in the 1830 edition, she was identified as the mother of God, not the mother of the Son of God. Joseph Smith made that change in the 1837 edition.
The change clarifies meaning for the modern reader, but was unnecessary for Nephi. Nephi already believed that God, Jehovah, was the son of the Most High God—or just abbreviated to the Son of God. Thus, for Nephi, the angel was presenting the mother of Jehovah in the flesh.
The original reading looks back to Nephi’s understanding. The 1837 clarification looks to the modern readers. The result, in verse 20, is the virgin holding the incarnate Jehovah as a baby in her arms.