It's real if the deck is not shuffled after every hand. Then you can count card with simple algo:
Low cards: 2-6 are counted as +1
Medimu cards: 7,8,9 are counted as 0
High cards: Ace, 10, K J, Q are counted as -1
So you count all cards on table, every hand, and then when the counting is high, you raise your bet amount.
But it's worthless at online casinos, because all shuffle deck every time.