An unlockable is a character, item, or stage that is not available from the start of a game. They are "locked" in a game, and have to be "unlocked" through certain means, usually a challenge of some sort, or by picking up a special item in a specific stage. Once they have been unlocked, they can be played with throughout the rest of the game.
Spin-off games usually have a large amount of unlockables, and have more unlockables than main-stream games. Unlockable stages usually have more gimmicks, or they are larger than the default stages, and are commonly more interesting than the default stages. Unlockable items may be more powerful than the default items. Unlockable characters may or may not be more powerful or better to use than the default characters, depending on their abilities.