A double bet is the most easy-to-understand multiple bet on offer. It's just two different bet selections combined into one bet. If your betting double succeeds, you win and receive a good payout better than you could expect from placing two separate single bets. A single bet is a bet made on one market event.
The strategy had the gambler double the bet after every loss, so that the first win would recover all previous losses plus win a profit equal to the original stake. Thus the strategy is an instantiation of the St. Petersburg paradox.