How does counting cards help you win at blackjack?

Card counters keep track of the "weight" of the deck, which lets them know the proportion of high and low cards left. This knowledge changes the odds on a given hand of blackjack. Decks that have a higher proportion of high cards are favorable for the player, since there is an increased chance of the dealer busting by drawing a 10. Thus, counting cards doesn't guarantee a win, but rather shifts the odds from being in the house's favor to the player's.