Online Casinos and Land Based Casinos (might) offer optional Side Bets to your actual Blackjack bet, which are Perfect Pairs and 21+3. You have to place a plain stake for your participating round to be eligible for punting Side Bets on your card combination. This optional bet is not a strategy by the actual definition, but a smart choice of the table to the right moment might increase the probability to win this special Game of Chance – secret information via PM.
You can win the additional bet nevertheless the outcome of the game round – even if the dealer should win by Blackjack. The difference in both specific Side Bets is depending on the card combination of your two cards only (Perfect Pairs) or the card combination of your two cards with the first open card of the dealer (21 + 3). You can choose if you pick one of both variations or both at the same time, as I usually do.