Every card in your hand is a card no one else can hold. That's card removal — or blockers: your two hole cards quietly remove specific combos from everyone else's possible range. Hold the A♠ and no opponent can have it, so there are fewer AA and AK combos left for them — you block the top of their range just by holding one of its cards.

The math is exact. A pocket pair like AA has 6 combos; remove one ace and only 3 remain — holding a single ace halves villain's aces. Offsuit-plus-suited, AK has 16 combos (four aces × four kings); hold one ace and it drops to twelve, hold a card of each rank and it drops to nine. Small removals, but they land on exactly the hands that matter most.

Why it matters depends on how narrow the range is. When villain's range is wide, removing one combo barely moves the needle. When it's narrow — a 4-bet, an all-in, a river range that's almost all value — blocking even one combo of the nuts meaningfully shifts the ratio of value to bluffs you're facing, and that ratio is what your call-or-fold decision hinges on. Blockers work on offense too: the best bluffs hold cards that block villain's calling range (bluffing with the A♠ on a spade board means villain is less likely to hold the nut flush), while your best value bets unblock it. "Bluff with blockers, bet for value with unblockers" is a solid first approximation.