We know from passages in the New Testament and, especially, from Latter-day revelation, that Elijah is one of the most important prophets to have lived. (In the Jewish tradition, he is second only to Moses.) Yet we know almost nothing about him. Why do you think that is?
In addition to the story of his life, in these and the next few chapters of scripture, we have Malachi’s prophecy that Elijah would come to bind the hearts of the fathers and the children (Malachi 4:5), as well as the repetition of that prophecy in several places, notably in D&C 2:1-3, where we are told that his coming will bring a restoration of the sealing priesthood. (See also D&C 110:13-16). The Savior thought the prophecy was so important that he repeated it during his ministry to the Nephites.
Of Elijah, Joseph Smith said:
The spirit, power, and calling of Elijah is, that ye have power to hold the key of the revelations, ordinances, oracles, powers and endowments of the fulness of the Melchizedek Priesthood and of the kingdom of God on the earth; and to receive, obtain, and perform all the ordinances belonging to the kingdom of God, even unto the turning of the hearts of the fathers unto the children, and the hearts of the children unto the fathers, even those who are in heaven. (Teachings of the Prophet Joseph Smith, page 337)
The Spirit of Elias is first, Elijah second, and Messiah last. Elias is a forerunner to prepare the way, and the spirit and power of Elijah is to come after, holding the keys of power, building the temple to the capstone, placing the seals of the Melchizedek priesthood upon the house of Israel, and making all things ready; then Messiah comes to his temple, which is last of all. (Documentary History of the Church, 6:254)