A ScareCard is a card that hits the board which makes people fear that someone has made a hand -- for example, a third or fourth flush card on the board, a card that would make a likely straight, or even just an ace added to the board.
Scare cards can be used as opportunities to bluff, but you have to be carefull that someone else didn't actually make the hand that you're bluffing!
Scare cards can be disastrous when you are on a SlowPlay if nobody makes a hand with them.