Also known as a card protector, card weight or even card cover, can be anything from a chip or a small toy which is placed on top of your cards when you re playing in a hand. 
This item protects your cards from being mucked or discarded by the dealer or another player accidently. In a game of poker especially when you are all-in and there are no chips in front of you, there are chances that a dealer might mistakenly take your cards and muck it which immediately kills your hand, you hand is pronounced dead.

To avoid such mistakes, a card protector is used.