Card counting in blackjack is a strategy where players track the ratio of high cards (10s, face cards, and Aces) to low cards (2-6) remaining in the deck. By keeping a mental tally of the cards that have been dealt, players can estimate when the deck is favorable to them, meaning more high cards remain. This allows them to increase their bets when the odds are in their favor and adjust their playing decisions accordingly. While not illegal, casinos monitor for card counters and may ask them to leave. It requires practice and is most effective in games with fewer decks and less frequent shuffling.