Only in the long run over hundreds of hours of playing can you count on winning. The underlying principle behind card counting is that a deck rich is tens and aces is good for the player, a deck rich in small cards is good for the dealer. The probability of busting a stiff hand is greater. The dealer is forced to hit a stiff hand and the player is not. To gauge the richness of the deck in good cards the player will keep track of the cards the are already played.
Strategies vary but all assign a point value to each card.
Everything else is 0, or neutral. Then the counter constantly adds and subtracts from the count according to the cards played. A positive count means that a disproportional number of small cards have already been played which means the deck is rich in large cards. This will tell you the relative richness of the deck in good cards. The true count is used in two ways, to determine how much to bet and how to play your hand.