Card holing in blackjack involves being able to see the hole card or the downwards facing dealer card. This happens thanks to a sloppy dealer who's not taking enough care to keep the down card hidden. For the player it can be very helpful in making decisions as to whether to bet high or low. The problem comes from the fact that you'd have to wager counter to perfect strategy. This will draw attention, as no one will understand why you made that particular game play. You may be asked to leave.
There are many ways that you an gain an upper hand when playing blackjack at the land based casino. They all fall under the title of advantage play. Essentially, you're finding ways to beat the casino's fixed advantage, also known as the house edge. The most well known method is card counting, though this is less relevant today than it was 20 years ago. Then there are things like trying to spot the dealers down card or marking the backs of the cards themselves. Finally, we have plain cheating.