The Song of Time (↓A→↓A→) is a song that has effects on the Temple of Time, or on time itself. In Ocarina, it opened the gateway between Link's present and the future. In Majora, it could be used to return to the morning of the First Day. And in Twilight, in which the temple has passed into memory, the song opens a gate into the past, allowing you to enter the temple.
Above person, you don't use the song of time in twilight princess, you just strike your sword into the Master Sword pedestal, and the statue blocking the door disappears