The Tree of Life is a tree that was once located in the Garden of Eden along with the Tree of the Knowledge of Good and Evil. The Tree of Life was located "in the midst of the garden." Adam and Eve were removed from Eden so they would not eat of the Tree of Life and live forever in their fallen state. God placed the cherubim and a flaming sword to "guard the way" to it after the fall.
The book of Revelation states that the Tree of Life is located in the center of the New Jerusalem and straddles the river of the water of life that flows from the throne of God and of the Lamb. It has twelve kinds of fruit and it yields its fruit each month. The leaves of the Tree will heal the nations.