Tree of Life is situated in the Garden of Eden, alongside The Tree of the Knowledge of Good and Evil. By eating the fruit, mankind could become immortal, because Adam and Eve were cast from Eden by God, mankind must suffer the consequences of death.
It is first referenced in the book of Genesis, where unlike the Tree of the Knowledge of Good and Evil, God does not prohibit Adam and Eve from consuming the fruit. According to Genesis, it is located within the "midst" of the Garden, along withe the Tree of the Knowledge of Good and Evil. In Genesis, it's purpose and function isn't defined.
It is also referenced in the last book of Revelation, this time with a purpose. It's leaves are described to be "for the healing of the nations," potentially for use in restoring the world to what God intends it to remain for eternity. Also in this book, it is described to have twelve kinds of fruit, of which it yields each month.