No reveal is not a new keyword mechanic coming out in Innistrad, it is a keyword action that has been around for ages. In Magic, it means the same thing as it does in English, to make known something that is not normally known. With Magic, that most often means showing the top card of your library, a card in hand, a card that you searched for in your library, or when a face down creature leaves the battlefield.

While revealing a card is pretty straightforward, there are two things to keep in mind. The first is that you can’t just flash the card quickly and hope that your opponent doesn’t notice what the revealed card is. You must completely show what the card is and give your opponent the opportunity to look at it if necessary.

The second is that failing to reveal a card when instructed to is a large and important infraction. At competitive and professional events the penalty is a game loss. And that’s only if you fail to reveal accidentally. If the judge believes that this failure to reveal was intentional the penalty is a disqualification.