An "accumulator" in this context, is the practice of betting on a series of races (or, actually any suitable events) where any winnings from the event are placed as a bet on the next event in the series. This means that winnings increase rapidly with serial success. Thus the term "accumulator" is not, in the dictionary sense really correct.
A colleague of mine, then just a printing apprentice, made several thousand pounds on an accumulator, starting with a £1.50 bet. Whilst he was very grateful, he wished, of course that he had bet a lot more.
