Card counting is a technique used in blackjack to keep track of the ratio of high to low cards remaining in the deck, allowing players to assess their advantage over the house. Players assign values to cards as they are dealt usually assigning positive values to low cards and negative values to high cards. By maintaining a running count and adjusting it based on the number of decks remaining (calculating the "true count"), players can determine when to increase their bets or play more conservatively.