Mask of Truth

The Mask of Truth is a mask featured in both Ocarina of Time and Majora's Mask. Its distinctive design prominently features a Sheikah Eye Symbol. The stone is a companion relic of two other items: the Lens of Truth and the Stone of Agony.

Ocarina of Time
The mask is obtained in Ocarina of Time by selling all of the sellable masks to various people all over Hyrule from the Happy Mask Shop. In this game, the mask has the power to allow Link to speak to Gossip Stones to learn new things about the world and its characters.

If Link wears the Mask of Truth at the Forest Stage in the Lost Woods before obtaining the Poacher's Saw, he will receive a Deku Nut upgrade. There is a glitch prior to the 3DS version that renders the upgrade unobtainable after that point.

Majora's Mask
In Majora's Mask, the mask imbues the wearer with several abilities, one of which is the ability to be able to read animals' minds, although this isn't a particularly useful skill except in the Doggy Race to see which dog might win. The second ability, which is also in Ocarina of Time, is the ability to talk to Gossip Stones. Notably, the item description for this mask is the only direct reference to the Sheikah made in the game.

The Swamp Spider House, which is located in Woodfall, must be completed to be rewarded with the Mask of Truth. Note that this requires Magic Beans (or Hook Shot, or Bunny Hood, alternatively), at least one bottle, the Sonata of Awakening, and the bow to complete.

Interestingly, Gossip Stones in Termina will give the young hero clues on how to obtain the mask; however, the clues are inaccessable if he doesn't have the mask yet.

Theory
The Mask of Truth is speculated to be part of a set of three objects, including the Lens of Truth and the Mask of Truth, that were created by the Sheikah tribe to give their holder untold power in terms of perception of the truth of their surroundings.