Most Christian accounts of a human life begin at conception. The Latter-day Saint account begins earlier. It holds that every person who has lived existed first as a spirit, begotten of Heavenly Parents, in a premortal world where character was already formed and choices were already made. Mortal birth is entry into a second estate, and the veil drawn over memory at birth is deliberate: a probation is not a probation if the candidate can remember the terms.
That premise reorders several familiar problems. Original sin is not inherited, because the second Article of Faith states that people are punished for their own sins and not for Adam's transgression; the Fall is treated as a necessary step rather than a catastrophe, and the Book of Mormon has Lehi say outright that Adam fell that men might be. Suffering in mortality is read as the condition of a test rather than as evidence against God's goodness. Death is a change of state within a continuing existence, not a boundary: the spirit world that follows is a place where the gospel is still taught and where those who never heard it in life can still receive it.
The sequence is set out most fully in the vision recorded as Doctrine and Covenants section 76, received by Joseph Smith and Sidney Rigdon at Hiram, Ohio, in February 1832, and in the sermons of Alma and Amulek in the Book of Mormon. Nothing in it is a private teaching. It is the framework taught to nine-year-olds in a Sunday class and drawn on a whiteboard by missionaries in the first or second lesson, and it is the reason a Latter-day Saint funeral tends to sound less like a farewell than like an interval.