A while back Greece had a problem. The problem was that it wanted to borrow more money, but didn't want to increase its debt. (Because the European Union would frown on it having more debt.) So it went to Goldman Sachs, and Goldman told Greece, well, what you can do is borrow some money from us, but we won't call it debt, because something something something something swaps.
This worked pretty well for everyone, for a while: Greece got the money, but nobody outside the deal understood that it had borrowed the money, because it was part of a derivative trade that was not accounted for as debt. Then it stopped working, and everyone got mad at Greece for disguising its debt, and at Goldman for helping Greece disguise its debt and charging rather richly for the service.