A staple is a card that is considered so splashable that it is included in almost any Deck the owner chooses to construct.
Opinions on what exactly constitutes a staple card often comes down to the individual, and today's current strategies and Deck-types have limited most staples to being mostly Spell/Trap Cards, such as "Dark Hole".
Most of these cards are (or were) included in Limited lists, mostly because of their usefulness. Many cards on the Forbidden list were once staples (e.g. "Pot of Greed", "Change of Heart", "Harpie's Feather Duster").