Card counting in Blackjack helps players improve their odds by tracking the ratio of high to low cards remaining in the deck. By increasing bets when the deck is favorable and adjusting play based on the cards left, a player can shift the house edge in their favor, potentially gaining an advantage of 1-2% or more. This strategy is effective over the long term, but it doesn't guarantee wins in the short term. Casinos often monitor for card counters and may ban players who are caught using this technique.