cards counting is statistic system that helps you to predict when the deck will give you high cards. the simplest example:
We will take one deck of playing cards, which contains 4 * 4 high cards which are:
Now lets say that there are 8 players that got all of these cards in one round, the next round there the chance that you will get 10 is 0, because none of them left in the deck.
In real world blackjack is played with 6 - 8 deks which makes it very hard to memorize what cards were pulled, In this system all of the decks cards are divided for 3 groups, based on the card value. this in porpuse to make it esier, so instead of counting the whole 14 cards we count only 3 in values of: -1 0 and 1, so the deck is divided into 3 groups: