From Wikipedia: Card counting is a strategy, primarily used in playing blackjack, to determine whether the next hand is likely to give a probable advantage to the player or to the dealer. Card counters attempt to decrease the inherent casino house edge by keeping a running tally of all high and low valued cards seen by the player. Card counting allows players to bet more with less risk when the count gives an advantage as well as minimize losses during an unfavorable count.
Thanks for the explanations, guys!