A splash is the practice of using only a very small amount of cards of a color in a deck which is filled with cards of other colors. The act of adding such variety to a deck is called splashing and may significantly improve certain match-ups at the cost of adding strain to the mana base as an additional color needs to be supported.
For example, after the release of Lorwyn blue/black Faerie decks became highly popular in Standard. As a response, the card Great Sable Stag, which was difficult to handle for these Faerie decks due to being uncounterable and also having protection from both colors the deck relies on, was printed in Magic 2010. To answer this new threat, Faerie decks splashed into red for another addition to the format: Lightning Bolt, an efficient removal spell that could handle Great Sable Stag as well as many other creatures.