Masks have been a common feature in The Legend of Zelda series since Ocarina of Time. When worn, the masks give Link a variety of abilities. Some of these abilities are required to complete the game, others are used for optional sidequests, and others are available just as fun items to collect.
Majora's Mask uses the masks as one of the primary gameplay features. Four masks allow Link to change his form entirely. While in another form, Link has access to abilities that are unavailable to him otherwise.