The first number is how much you win relative to the second number, your bet. 1:1 means you win $1 for every $1 you bet. Place a bet for $10, and win $10, meaning you receive $20 (your bet + your winnings). 3:1 means if you bet $10, you'd win $30, for a payout of $40. 1:5 means if you bet $10, you'd win $2, and get paid $12.