Card counting is a type of advantage play you can use to increase your winning chances. It involves keeping a count in your head to determine how likely a specific hand is to come up.
Although not illegal, many casinos disapprove of this technique and may ask you to leave if you get caught counting cards.
For example, if a casino floor suspects you’re counting cards, they might ask you to leave, or back you off from playing blackjack at their property. In the latter case, you’ll still be allowed to play other games.