Computers have calculated that a one deck game where the dealer stands on any 17 and the player can split any pair and double on any first two cards is an even game. They have also calculated that with one card removed, the advantage shifts by a percentage in favor of either the player or the dealer.
In a multi-deck game, the dealer starts with a small advantage, and the percentage of the cards as they are removed is not as great as in a one deck game, but there is still an effect. Counting is simply keeping track of the swing of the odds as the game is played to help you decide when to change your bets. By keeping track of an equal number of cards from both the +group of cards and the - group of cards you can decide when to increase your bets. You do not keep track of every card played, just groups of cards.
There are an equal number of 3 to 6 cards as there are 10 to King cards. It is quite simple to know when there are more 10s than normal. This means you can know when you are most likely to win!
