To understand card counting you have to have some knowledge of blackjack. The basic idea is that you want the cards in your hand to add up to be as close as possible to 21 without going over 21. Face cards count as 10 and the ace can count as either 1 or 11. Everyone is initially dealt two cards and you can request as many additional cards as you want.
Everyone plays against the dealer. If you are closer to 21 than the dealer without going over then you win. If you are over 21 then you lose. Otherwise the dealer loses if over 21. The dealer deals themselves another card if and only if the total is less than 17.
The basic idea of card counting is to keep track of the number of 10’s and face cards in the deck. If there are a lot remaining in the deck then the odds favor the player over the dealer, because the best strategy under these conditions is to not ask for additional cards even with low counts and hope that the dealer goes over, which is fairly likely. When the count is in your favor you make a high bet and when the odds are against you, you make a minimum bet.