It's not money they owe, but money owed to them; backed by some form of contract. Your credit card contract for example.
Person A owes person B money, let's say $100. Person B sells that debt to Person C for $80.
Person A still owes the same $100, just to a different person.
Person B gets paid now instead of waiting for Person A to pay up, but lost $20 on the deal.
Person C invested $80, hopefully gets paid the full $100, not having to chase down a delinquent debtor, and profits $20 out of the deal.
Nobody wants to be person C when it comes to the Debt Twitter owes.
If you had someone that owed you money (or anything else really), you could sell that debt, assuming you can find a buyer for it. Banks just do this on a much larger scale than $100 at a time.
Say you have a credit card debit and you take out a new card because it has a free credit transfer offer going on. You're selling your debt right there.
Another scenario is you have a loan and refinance it using a new loan.
Might be? This is the worst investment they ever made, and they know it, mostly because they've speaking publicly like this, because usually they bury the lead - to protect their other investments.