A blocker is a poker term for a card that a player has in their hand that makes it less likely or even impossible for their opponent to have a specific combination.
Effectively using blockers can help you obtain a small edge over other players and find profitable bluffing and hero calling opportunities.
For example, if you have pocket tens and the board reads 3, 7, 8, 9, K, you have a solid blocker that reduces your opponent’s chances of having a straight, as you hold two of the four tens cards in the deck.
Another common example is having a blocker to a nut flush. If you have Ah Kd on the river on a three-heart board like Jh Th 6d 2h 3s, you know that your opponent does not have the best possible flush and can use this poker hand to bluff effectively in many situations.