When no one believes a country can pay back its debts, people stop buying its bonds. And if that happens, it starts to mean the currency or money is worthless because the markets stop believing it has value. The belief that the US was a stable country meant we could basically spend what we want.
npr.org
The bond market is signaling trouble ahead. This is why you should pay attention
Many people don't pay attention to bond markets, but they should — especially now. This is why they matter and why the drop in bond prices is signaling investor concerns about the U.S. economy.