Counting cards at a blackjack table online where the outcome is determined by a Random Number Generator (RNG) is just as useful (useless) as counting cards at a blackjack table in a brick-and-mortar casino where an automatic shuffling machine is bringing discarded cards back into play after each round.
Counting cards online will only be a good idea if you manage to find a live dealer casino where discarded cards are not shuffled back into play after each round.
What is card counting?
In the context of blackjack, card counting is the practice of paying attention to which cards that have already been discarded from the shoe, in order to know which cards that are still left in the shoe.
For most of us, remembering each discarded card would be an overwhelming task. Instead, card counters at the blackjack table tend to use systems that focus on certain cards, e.g. the aces and the 10 point cards. How “player friendly” the remaining cards in the shoe are deemed to be will then guide the player’s betting decisions, e.g. how many boxes to play, how large wagers to make, when to stay and when to hit, etc. Skilled card counters will even use the information to purchase insurance or accept even money on blackjack in certain situations.
As you understand, trying to keep tabs on discarded cards is pointless if you are playing at a blackjack table where the dealer is putting discarded cards into the shuffling machine. Card counting is only useful at tables were an old-fashion “shoe” is used and the dealer only shuffles the decks once in a while. Card counting is also carried out at single deck blackjack tables, although a system is rarely necessary there since there is only one deck to memorize.