Nephi continues his condemnation of the people. They have rejected the fundamental belief in the coming Atoning Messiah, and they have exchanged the god of materialism for the God of their fathers.
Referent: The words we have from Nephi here are obvious mirrors for Matthew:
Matthew 6:19-20
19 ¶ Lay not up for yourselves treasures upon earth, where moth and rust doth corrupt, and where thieves break through and steal:
20 But lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through nor steal:
Nephi certainly had this concept in mind where there was a contrast between the eternal treasure of heaven and the temporary treasures of the world. However, the specific language Joseph Smith used was obviously colored by Joseph’s knowledge of the King James Version of this text. There are obviously some differences as well, so Joseph is not copying from Matthew, but rather being influenced by his memory of the passage that is parallel to the one that he is translating.