Introductory Note: The scriptures referenced in this message refer to 'sons' a great deal. As it is my intention to encourage readers to liken these verses to themselves, please substitute the word 'daughters' or 'children' as desired.
Malachi 3:1-3
1 BEHOLD, I will send my messenger, and he shall prepare the way before me: and the Lord, whom ye seek, shall suddenly come to his temple, even the messenger of the covenant, whom ye delight in: behold, he shall come, saith the LORD of hosts.
2 But who may abide the day of his coming? and who shall stand when he appeareth? for he is like a refiner's fire, and like fullers' soap:
3 And he shall sit as a refiner and purifer of silver: and he shall purify the sons of Levi, and purge them as gold and silver, that they may offer unto the LORD an offering in righteousness.
Near the end of the old testament Malachi foresees that the Lord will send a messenger to prepare the way before he suddenly comes to his temple. He then asks who will be able to abide the day of his coming. The answer: those 'sons of Levi' who have been refined and purified by the Lord, and who have prepared for him a righteous offering. As you probably know, Levi was one of the 12 sons of Jacob/Israel. His descendants were the priests who performed ordinances on behalf of the children of Israel. (That's important to remember for later.)
Doctrine and Covenants 2:1-3
1 BEHOLD, I will reveal unto you the Priesthood, by the hand of Elijah the prophet, before the coming of the great and dreadful day of the Lord. 2 And he shall plant in the hearts of the children the promises made to the fathers, and the hearts of the children shall turn to their fathers. 3 If it were not so, the whole earth would be utterly wasted at his coming.
In the very next chapter (which is quoted by Moroni to Joseph Smith with some alterations) Malachi reveals that Elijah is the aforementioned messenger and he elaborates on Elijah's mission: to emphasize the familial nature of and blessings available to those who make temple covenants with the Lord. Ensuring that we have the opportunity to make and keep these covenants is the whole purpose behind the creation of the earth and our mortality on it.
Fast-forward to September 1832 when Joseph Smith received a revelation on Priesthood containing what we refer to as "the Oath and Covenant of the Priesthood."
Doctrine and Covenants 84:33-39
33 For whoso is faithful unto the obtaining these two priesthoods of which I have spoken, and the magnifying their calling, are sanctified by the Spirit unto the renewing of their bodies.
34 They become the sons of Moses and of Aaron and the seed of Abraham, and the church and kingdom, and the elect of God.
35 And also all they who receive this priesthood receive me, saith the Lord;
36 For he that receiveth my servants receiveth me;
37 And he that receiveth me receiveth my Father;
38 And he that receiveth my Father receiveth my Father's kingdom; therefore all that my Father hath shall be given unto him.
39 And this is according to the oath and covenant which belongeth to the priesthood.
Malachi referred to 'sons of Levi'. Now the Lord refers to 'sons of Moses and of Aaron'. Who were Moses and Aaron?
Exodus 6:16-20
16 And these are the names of the sons of Levi according to their generations; Gershon, and Kohath, and Merari: and the years of the life of Levi were an hundred thirty and seven years.
17 The sons of Gershon; Libni, and Shimi, according to their families.
18 And the sons of Kohath; Amram, and Izhar, and Hebron, and Uzziel: and the years of the life of Kohath were an hundred thirty and three years.
19 And the sons of Merari; Mahali and Mushi: these are the families of Levi according to their generations.
20 And Amram took him Jochebed his father's sister to wife; and she bare him Aaron and Moses: and the years of the life of Amram were an hundred and thirty and seven years.
Moses and Aaron were 'sons of Levi' in a direct, familial sense. When we receive priesthood ordinances and make covenants with the Lord we become the 'sons of Moses and Aaron' (who we have established as 'sons of Levi'), and especially as we help others (our deceased family members) receive priesthood ordinances we act in a capacity very similar to those ancient 'sons of Levi'.
What is the 'offering in righteousness' spoken of by Malachi?
In a letter from Joseph Smith to the church in September 1842 (ten years after D&C 84 was received) Joseph puts it all together:
Doctrine and Covenants 128:24
24 Behold, the great day of the Lord is at hand; and who can abide the day of his coming, and who can stand when he appeareth? For he is like a refiner's fire, and like fuller's soap; and he shall sit as a refiner and purifier of silver, and he shall purify the sons of Levi, and purge them as gold and silver, that they may offer unto the Lord an offering in righteousness. Let us, therefore, as a church and a people, and as Latter-day Saints, offer unto the Lord an offering in righteousness; and let us present in his holy temple, when it is finished, a book containing the records of our dead, which shall be worthy of all acceptation.