What is an accumulator?
An accumulator (“acca”) combines several selections into a single bet, rolling each leg's winnings onto the next. Every leg must win for it to pay out, but the combined odds multiply with each selection — which is how a small stake can return a large amount.
Example
Four football teams to win, each at 2.0 (evens): £10 returns £160 if all four win (2×2×2×2 = 16× the stake). One losing leg and the whole acca loses.