Blackjack card counting is a strategy used by players to gain an advantage over the casino by keeping track of the ratio of high cards to low cards that have been dealt. By keeping track of the cards that have been dealt, players can gain an advantage by betting more when the ratio of high cards to low cards is in their favor. Card counting is not illegal, but it is frowned upon by casinos and can result in the player being asked to leave the premises.
Card counting is not illegal in most jurisdictions, but it is considered cheating in some casinos. Casinos may ask players to leave if they are suspected of counting cards, and they may ban players who are known to be card counters.