Alice is broke and in need of $100 to buy food. She goes to Bob and tells him about her problem.
Bob is actually broke as well, but he keeps quiet about that and presents his idea:
"You and I have never done business together before and owe each other nothing. However, let's pretend that I owe you $100 and you owe me $100 - that still keeps us even."
Bob takes a piece of paper from his desk, writes $100 on it and signs it.