In the previous verse, Nephi saw a child in the arms of his mother. The angel proclaims his identity. Even though this is the child in mortality, it is the eternal, transcendent titles by which he is identified. One is that he is the “Son of the Eternal Father.” As with the “mother of the Son of God” phrase in verse 18, this is the result of Joseph Smith’s editing in 1837. It originally read “even the Eternal Father.” The reason for the change is the same. The original displayed Nephi’s understanding, and the 1837 change reflects more modern understandings.
The interesting thing is that as soon as the Lamb of God is shown, then the angel asks if Nephi knows the meaning of the tree. The angel apparently expected that the vision of the mortal Messiah would clarify the meaning of the tree. It does, but why?
One of the stories with which Nephi would have been familiar was of the goddess Asherah as a celestial wife of God. The oldest versions of the story had her as the wife of the Most High God, though later she was wife to Jehovah. That relationship creates a very close tie between the woman and the child, and the celestial mother and her divine child.
The connection between Asherah and a tree was an important part of the worship of that goddess, and her counterparts, in the ancient Near East. Thus, seeing a tree associated with the mother of a deity would have been a natural connection for Nephi. By seeing Jesus and his mother, the nature of the tree shifted from one that bore fruit that gave joy to the very person who authored and enabled that joy for God’s children on earth.