Blackjack: Card Counting
In card counting, a player keeps a mental count of the relative number of high cards to low cards that have been played.
High cards are considered everything that can be a 10 or 11, or the 10-Jack-Queen-King-Ace cards. 
If there are more high cards in a deck, that’s to the player’s advantage. If there are more low cards in the deck, that’s to the dealer’s advantage.
Card counting doesn’t assure a blackjack player of winning.
The problem with card counting is, if you don’t count the cards right, then you’ll end up betting the maximum bets when you shouldn’t.
You’ll need to learn the basics of card counting. Finally, you’ll need to master card counting in a casino environment, where there is a dealer and other players and a room full of bright lights and strange sounds. This can be done, but it will take time.
Don’t consider trying strategy variations based on your card count until you know for certain you’re counts are accurate.