It refers to having to pawn one thing in order to pay for something else. To 'pop' was a slang word for pawning something. So, in the nursery rhyme when it says: 'that's the way the money is spent, pop goes the weasel' it means that in order to spend your money you have to pawn something else. Oh, and by the way, a weasel was a slang term for an iron.