A 2, 3, or 12 will double your money on a come out roll if you've placed a Don't Pass bet. Rather than hoping for a 7 or an 11 on the come out roll, you're hoping for a 2, 3, or 12 (the losing roll of Pass Line bets).
Pass Line bets are also said to be 'betting right,' while Don't Pass bets are said to be 'betting wrong.' (Not that either is any better or worse a bet than the other - this is just craps jargon.) Don't Pass bets are just the opposite of Pass Line bets. Placing a Pass Line bet is betting with the dice, and placing a Don't Pass bet is betting against the dice. If, after a point is established, a 7 is rolled before the point number is rolled again, you also lose.
If the number rolled on the come out roll is a 2, 3, or 12 (called Craps), you lose.
