At it's simplest card counting involves keeping track of how many hard 10s (10, k, q, j) there are in the shoe, assuming you know how many decks are in the shoe and that the discard isn't reshuffled into the shoe too often. When you find yourself in a situation where there are a lot of tens still in the deck but a lot of the smaller value cards have been played, you bet heavily and stand on low values hoping that the dealer will tend to bust (in most casinos a dealer will hit on 15 or below and stand on 16 or above, more tens in the deck means it's more likely that the dealer will bust, yielding a small statistical advantage to the player in a tens-heavy scenario)