I was watching Hudson's and Desai's video on money and the dollar system, where Hudson mentions that money is debt, but I didn't understand the concepts. Then I found this video explaining it more succinctly, with cartoons and historical development and such, but I still don't understand it at all.

I'm looking for extremely basic introductions on the matter - how exactly did debt connect with exchange of commodities? How did that develop into what we have now? What does a bank really do behind the curtains?