Door of Time

The Door of Time blocks access to the pedastal which holds the Master Sword. The door is in the Temple of Time and can only be opened by collecting together the three Spiritual Stones: the Kokiri Emerald; the Goron Ruby; and the Zora's Sapphire. When these three stones are brought together and the Ocarina of Time plays the Song of Time, the door shifts and the path to the legendary sword opens.

According to Rauru, the Door of Time is, effectively, an obstacle to the Sacred Realm put in place when the Triforce was sealed away. At the start of The Legend of Zelda: Ocarina of Time, Ganondorf is attempting to remove this obstacle but is frustrated by the guardians of the Spiritual Stones.