A double involves two selections. If the first selection wins, what would’ve been the return in a single bet instead all goes onto the second selection as the stake. To gain a return this second selection also has to win. If the first selection in the double bet loses then the double also loses. Where the odds of one or both selections are large enough it may be worth doing an each way double. In that scenario half of the stake would be on the win bet, and half on the place component, so if both parts of the bet placed, there would at least be some return.
A single bet is the most straight forward bet there is, and of course it’s not an accumulator bet. It involves placing a bet on a single outcome, such as a horse winning a race or a particularly football score coming up. It’s a stake placed on an outcome without factoring in any other additional outcomes or bets (in contrast to a doubles, trebles or acca bet). Your selection needs to win in order for you to gain a return.