The priests with the correct authority from the Son are to teach the commandments of the Son to the people so that they might enter into his rest. Here Alma ties up his themes. He has spoken of the plan of redemption whereby mankind could be redeemed from the temporal and spiritual death. He has indicated that the spiritual (second) death comes from the choices men make, and therefore God had to provide the commandments to show the way.
He now has addressed the human chain through which those commandments are transmitted. The high priesthood has its authority through righteousness, and part of its responsibility is to teach these commandments of the Son, in order than men might be able to choose good, and follow the plan of redemption.