Card counting systems in blackjack are strategies designed to track the ratio of high to low cards remaining in the deck, helping players determine the optimal times to increase bets. These systems assign values to certain cards, typically giving low cards a negative value and high cards a positive value. As cards are dealt, players keep a running count based on the assigned values. A positive count indicates that more high-value cards are remaining, suggesting a favorable situation for the player, while a negative count indicates the opposite.