Bonus Bets are bets that do not use the bettors actual money and instead are free bets that the Sportsbook has given the bettor.
When you place a bet using a Bonus Bet the winnings are calculated differently to a cash bet. A bet placed with a Bonus Bet will pay out the winnings minus the Bonus Bet stake. For example, if you placed a $20 Bonus Bet at odds of $10 your winnings would be $180.