If you're a card dealer, you know what this is. It's a card, usually made of plastic, which is kept on the bottom of the deck to conceal the bottom card and also ensure the players you're not dealing from the bottom of the deck.
These cut cards are made from 1/16" Maple or Mahogany wood and cut so the grain runs the length of the card. That makes the card pliable so it fits the curve of your hand as you hold the deck. Several wood varieties are available.