Card counting is an advantage to play with technology. This is a way to track cards by assigning values to certain cards in the deck. The theory behind card counting is based on the fact that when a deck lacks a small card, the player's card advantage exceeds the normal advantage. When the deck or card lacks a high card, the player's card is lower than the normal value.
While card counting is a technique that can increase your chances of winning at blackjack, it can't be applied to online blackjack sites that are shuffled every time and have no deck penetration.