Japan owns a lot of US debt. If they, or especially China dumps their US dollar investments, we are in Big(ger) Trouble (than we're in now).
Well, let’s look at the real world instead of paper.
Stocks and bonds are deferred consumption. People save and invest money on the assumption that they will eventually be able to buy real goods and services in the future.
Japan’s problem is that the amount of outstanding debt so large that they cannot tax enough real goods and services out of their economy to satisfy what is owed to the bond-holders, if they eventually decide that they want to trade in their holdings for physical goods and services, as is likely enough as they approach old age.
Now Japan does have a lot of US Treasury debt. But if they just sold it, and converted the proceeds into yen, they still couldn’t supply physical goods and services to their bondholders. They would be better off to hold the bonds, or sell them for dollars, and use the money to buy goods from the US. We do have physical capacity and could use the business, to some extent. We would have to consume somewhat less, but money from overseas would be flowing into our economy.
Our problem has been that our bondholders have been willing to hold paper certificates, and paper dollars. That takes away the discipline that we need to have a proper economy, because we can overconsume just by issuing debt. Of course, if the dollar is a world reserve currency, foreign countries will just trade back and forth among themselves using dollars. But each dollar overseas in fact represents a potential demand on physical goods and services from US producers.