Keys are items from the Mario series that first appeared in Super Mario Bros. 2 and are used for unlocking doors. Often, when Mario picks up a key, he is chased by one or more Phantos. They are commonly found in puzzles under Warp Pipes. In Super Mario World, they provided access to secret exits from each level, sometimes unlocking extra levels.
In Luigi's Mansion, keys are accessed upon completion of a specific area of the mansion.
In Super Mario 64 DS, bunnies can be found holding keys which unlock drawers in the Rec-room and unlock new minigames.