In Blackjack, unlike in other gambling games, such as craps or roulette, a deal has an influence on the next one. 
The cards are dealt from a deck so the remaining deck is not complete. When one observes the cards that have already been dealt, he will be able to assess whether the remaining cards may be advantageous for him. When the cards to be dealt are not advantageous for the player, he may assume that his hand won’ be good.
It’s very difficult for a player to remember all the cards in Blackjack that have been dealt, so a card counting system is established to make it easier. All you have to be able to do is to add and subtract by units of one since all the cards are given either negative (-1), positive (+1) or zero value.
There are many counting systems but usually low cards, such as 2 or 3 are counted as +1 since they have a positive value. High cards have negative value (-1).
You must have to remember that the lower values you take, the easier it will be to remember for you. When you learn to count cards, you will notice that it gives you great possibilities and you can base your bets on the counting. Your play will be much safer with the strategy since you can make safer choices if you know more about the card countig. This is a very accurate strategy.