Credit is an abstraction over
future resources. When you take out a loan, you're basically saying "I am betting that I can produce enough in the future to pay back the resources that I am using now." And if you don't, you're on the hook for it, and potentially go bankrupt.
That's why the financial sector is the weak link, and also why it's very well-compensated. Finance is a future-prediction problem: you do well if you successfully predict which investments will pay off, and you go bankrupt if you fail. Predicting the future has always been hard.
The reason we get these boom/bust crises is because instead of using their own judgment, many people, when faced with a hard problem, will just do what everyone else is doing. So the whole financial industry lemming-trains into bad investments, and then when they bust, they all go bankrupt and there's no resources to go around until the wreckage has been cleared away and placed in more independent manager's hands.