Forbidden fruit

The Pearl of Great Price has an account of the creation of the earth and the fruit-bearing trees placed in the Garden of Eden by the Lord:

And I, the Lord God, planted a garden eastward in Eden, and there I put the man whom I had formed (Moses 3:8).

...even all things which I prepared for the use of man; and man saw that it was good for food. And I, the Lord God, planted the tree of life also in the midst of the garden, and also the tree of knowledge of good and evil (Moses 3:9).

And I, the Lord God, took the man, and put him into the Garden of Eden, to dress it, and to keep it.

And I, the Lord God, commanded the man, saying: Of every tree of the garden thou mayest freely eat,

But of the tree of the knowledge of good and evil, thou shalt not eat of it, nevertheless, thou mayest choose for thyself, for it is given unto thee; but, remember that I forbid it, for in the day thou eatest thereof thou shalt surely die (Moses 3:15-17).

The fruit of the tree of knowledge of good and evil was the "forbidden fruit." By placing it in the garden, the Lord created for Adam and Eve, opposing choices with consequences attached.

And to bring about his eternal purposes in the end of man, after he had created our first parents, and the beasts of the field and the fowls of the air, and in fine, all things which are created, it must needs be that there was an opposition; even the forbidden fruit in opposition to the tree of life; the one being sweet and the other bitter (2 Nephi 2:15).

For it must needs be, that there is an opposition in all things. If not so, ...righteousness could not be brought to pass, neither wickedness, neither holiness nor misery, neither good nor bad (2 Nephi 2:11).

Without offered opposites, Adam and Eve could not make choices, and the order of God is to guarantee free agency to man. "A major purpose of earth life is for God’s children to prove “if they will do all things whatsoever the Lord their God shall command them” (Abraham 3:25). In order to prove their willingness to do all things God would command them, it was necessary that Satan be allowed to tempt man or to provide the necessary opposition to God so that choice would be possible. A knowledge of good and evil is gained only in a world of opposites, where wickedness opposes righteousness; good, bad; corruption, incorruption; happiness, misery; bitter, sweet (2 Nephi 2:11–12). Because Adam and Eve ate the forbidden fruit, their eyes were opened to a world of opposites. The Lord himself declared, “Behold, the man is become as one of us to know good and evil” (Moses 4:28) (Lesson 6: The Fall of Adam,” Aaronic Priesthood Manual 3, 19).

The doctrine of The Church of Jesus Christ of Latter-day Saints holds that there was no "original sin" which taints all men born on this earth. Adam and Eve transgressed a commandment of God, but they did not commit sin. They fulfilled a requirement of the Plan of Salvation. By leaving the Garden of Eden, they were able to fulfill another of God's commandments, the commandment to procreate, which they could not do in their paradisaical state in the Garden of Eden. They also brought physical death into the world, which paved the way for resurrection and the possibility of Eternal life. The atonement of Christ overcame both physical death and spiritual separation from God. Thus, the Fall was an integral part of God's plan of love for mankind.