A double is a multiple bet in which a single stake is wagered on two selections. Both of these selections must win in order for the bet to pay out.
Example:
A punter places a $5 double bet on:
Manchester United winning the Premier League at 3/1
Chelsea winning the Champions League at 5/1
If Manchester United wins the Premier League, the $20 payout will be automatically wagered on Chelsea winning the Champions League at odds of 5/1.
If Chelsea fails to win, the $20 stake is forfeited and the punter loses the bet. If Chelsea wins the Champions League a return of $120 is generated and paid out to the punter.