A side bet on the poker hand made by your two cards and the dealer's upcard.
It pays when your first two cards and the dealer's up card make a flush, straight, three of a kind or straight flush. Example: you hold 7♥ 9♥ and the dealer shows 2♥, a flush. The paytable sets the cost: with 6 decks, all 9 to 1 is 3.24%, a table paying 25 for trips is 7.14%. It costs more than the main game (under 1%), so don't bet it.