This poker term has several meanings:
(1) To make a bet in order to force opponent(s) to pay in order to draw to the best hand. For example, if you have two pair and there is a flush draw out there, you want to bet big in order to protect your made hand that you think is likely the best hand, and to give your opponent incorrect odds to make the call, forcing your opponent to make a mistake and to get value from drawing hands. When used in this instance, it’s often referred to as a “protection bet”.
(2) Also, physically protecting one’s cards while the hand is in play using a card protector like a poker chip so that the dealer cannot accidentally collect and muck the cards by mistake.