The Triforce of Power is one of three pieces of the Triforce, the other two being the Triforce of Courage and the Triforce of Wisdom.
When Ganondorf infiltrated the Sacred Realm and attempted to take the Triforce, he only got the Triforce of Power because his heart was unbalanced; the Triforce of Courage and Triforce of Wisdom went into the possession of Link and Zelda, respectively.
When Link reaches Ganondorf's tower chamber at the top of his castle, he uses the Triforce of Power to try and take the Triforce of Courage from Link, without success. However, his powers prevent Navi from going near him, thus, she cannot target Ganondorf unless he is stunned with a Light Arrow. After Link and Zelda escape from Ganon's Castle when Ganondorf causes the castle to collapse in an attempt to kill them, he uses the Triforce of Power to transform into his beast form, Ganon, and battle Link again. With Zelda's help, Link defeats Ganondorf, and he is sealed away in the Evil Realm by Zelda and the sages, though he vows to kill their descendants as long as he has the Triforce of Power when the seal is broken.