power sword

By that he means that you need to do another quest to get it to full power.
'The fabled Master Sword works exactly like the Hero's Sword, only that it's got bigger reach and causes more damage. The Master Sword will automatically undergo two upgrades as you progress through the quest. Each upgrade makes the sword more powerful and changes the look of the hilt. Only when the sword is fully upgraded through the power of the two sages can it harm Ganondorf.'

What you have to do is: Go to the Earth Temple ( that is to the right of Outset Island). Once you are on the island, you will see a giant rock that is shaped like a guy's face. You can't lift it or bomb it so you have to go fire island and go inside and get the power bracelets and only then can you lift the giant head. But if you're wondering" How the heck do I get inside?" Well, you have to warp to the Mother & Child isles and confront the fairy. Then, go to the fire volcano island near Dragon Roost island. Shoot it with an ice arrow, go inside, get the power bracelets, go to the head island, lift the head and go inside. Talk to the sage after learning the Earth God's Lyric, she will tell things about Ganondorf and also that he can restore some of the power to the master sword. But, you will have to go find someone on the great sea with the same musical instrument as her. For the rest of the answer refer to the walkthrough.