There were some prophets who had risen up after Lehi and his family left Jerusalem. One of these was Malachi who lived around 400 B.C.
This verse is referring to John the Baptist who arose among the Jews to prepare them for the coming of the Savior. Malachi promised that when Jesus came he would bring a whole new covenant to replace the law of Moses and the covenant with Abraham. The new covenant would encompass the entire gospel of Jesus Christ.