The entire world is roughly 225% leveraged to its economic output. Emerging markets are a bit less and advanced economies a little more.

But regardless, everyone's "real" debt is likely much bigger, since the official totals miss a lot of unfunded liabilities and other obligations.

Debt is an asset owned by the lender. It has a price, which—like anything else—can go up or down. The main variable is the lender's confidence in repayment, which is always uncertain.

But there are degrees of uncertainty. That's why (perceived) riskier debt has higher interest rates than (perceived) safer debt. The way to win is to have better insight into the borrower's ability to repay those loans.

If a lender owns debt in which his confidence is low, but you believe has value, you can probably buy it cheaply. If you're right, you'll make a profit—possibly a big one.