The tree of life stood in the middle of the garden of Eden, its fruit offering unending life in fellowship with God. When Adam and Eve fell, they were barred from it, lest they eat and live for ever in their fallen state — and the way to the tree was guarded by cherubim and a flaming sword. Access to life was lost.
But the tree reappears at the very end of the Bible, in the new Jerusalem: on either side of the river of life stands the tree of life, and the leaves of the tree were for the healing of the nations. What was lost in Eden is restored and surpassed. Proverbs even calls wisdom, and the fruit of the righteous, a tree of life — foretastes now of the endless life that awaits God's people in his presence.
The Tree of Life in Scripture
In the midst of the street of it, and on either side of the river, was there the tree of life, which bare twelve manner of fruits, and yielded her fruit every month: and the leaves of the tree were for the healing of the nations.