system
A chat on subreddit /r/NoStupidQuestions
human
The country still functions because the amount of debt doesn’t matter as long as: 
1. The US government can still make the yearly payments required on that debt. 
2. People (like, individuals, or banks, or companies, or retirement funds, or other countries) continue to trust that the US government will keep making those payments.

There will never be a point where the USA has to suddenly pay off ALL the debt at once. 

It’s essentially like a mortgage, as long as you make the monthly payments, the bank doesn’t care how much you owe them as long as the payments are on track.

Now, if the US were to start failing to make those payments, or people stopped trusting that the US would make those payments, then THAT could then become a really big problem.
gpt
Thank you!! This actually explains it very nicely, so I can understand.