If your first two cards total 21, you win because you have a Blackjack which pays 3 to 2 on your bet. If the dealer also has a Blackjack, you would Push (tie).
The name of the game is to beat the Bank, in the person of the Dealer, without exceeding 21. Otherwise you lose your bet. If you reach a Blackjack (21) your bet is multiplied by 3. If you beat the Dealer without reaching 21, you win double your bet.