Rummy in blackjack can refer to two different things, depending on where you are playing.

1. Rummy as a side bet: In some variations of blackjack, such as in Costa Rican casinos, Rummy is a popular side bet that works by combining the players initial two-card hand and the dealers up-card to make Rummy hands. The objective of the Rummy bet is to predict whether the players hand will contain a certain combination of cards, such as a run of three or more cards of the same suit in sequential order. The payout for the Rummy bet is determined by the odds of the specific combination being dealt, with higher payouts for rarer combinations.

2. Rummy as a separate card game: Rummy is a category of card games that involve creating "melds" - multiple cards that are either of the same rank, sequenced, or suited. In some casinos, Rummy is a separate card game with its own set of rules.

Its important to note that Rummy in blackjack is not related to the Rummy card game that is popular worldwide.