When you use a card for attack, defense or for damage bonus, you must exert it, meaning that it cannot be used again until after your next Ready Phase. This is shown on the game board by turning the card red. Some cards have special abilities that allow them to be readied prior to your ready phase. This will be noted in their game text. See Hunker Down for an example. In other CCG's, such as Magic: The Gathering, this is called tapping.