The flexibility of a card is defined by its ability to use it within different circumstances. Choosing flexible cards or building flexibility into the structure of your deck will lower your risk and increase your consistency.
Magic is a game ruled by possibilities. The more possibilities your cards can offer you, the more flexible you will be when playing your deck, and that flexibility often makes the difference between winning and losing. An inbuilt hatred in the card towards certain other cards may cause it to become a preferred card in the sideboard. Cards with several abilities also offer flexibility, and cards that may be used at the speed of instants increase the possibilities of responding well to an opponents actions.