Blockers
A blocker is a card in your hand that reduces the number of combinations of a specific hand your opponent can hold, most often the hands that would call or beat you.
Every card you hold is a card your opponent cannot have, and that single fact shrinks the pool of hands left for them. A blocker uses that shrinkage on purpose: holding a card that is part of your opponent's strongest possible hands makes those hands less likely, which is why certain bluffs with zero showdown value work better than others. The same idea runs in reverse for thin value bets, where holding a blocker to the hands that would call you is a reason to check instead.
Worked example. Before any cards are removed, Ace-King exists in 16 combos. Hold one of the four aces yourself and only 3 aces remain in the deck, so your opponent's possible Ace-King holdings fall from 16 combos to 12. Do the same with a pocket pair: hold one queen and the 6 combos of pocket queens drop to 3, because only 3 queens are left to pair up.