"Peeking for blackjack" is the practice of the dealer checking their face-down card when their upcard is an Ace or a ten-value card. This is significant because it can determine the outcome for players; if the dealer has a blackjack, players lose their bets unless they also have one, resulting in a push. This potential influences players' strategies, making them more cautious if they sense the dealer might have a strong hand or more aggressive if the dealer does not.