Where the existence of something necessarily produces more of it, in an exponential way.
GDP is $23T and growing at 3%
Debt is $32T and growing at 6%
The debt is paid from GDP at a service rate of 5%. See the problem?
At some point in the future even 100.00% of GDP will not be enough to service the debt.
National debt is benign in a lot of conditions, but if you lose control of it like this, you get locked into an inescapable malthusian catastrophe.