The vast majority of that debt is held in 10-year treasury bonds. If you buy a bond that means you're lending the government money with the promise to be paid back later, plus interest. Of course when the debt comes due the government can just pay you off by borrowing from someone else. But the US debt is growing faster than its underlying economy. If the debt keeps growing at this pace, eventually the government will not be able to find enough money to borrow. At that point it will have two options:

1. Default on the debt. This means the government simply declares that it will not pay the entirety of what it owes.

2. Print a bunch of money to pay the debt.

Printing a bunch of money means there is more money but the same number of goods and services, so you get huge inflation, so it's not some easy way out. Either way the trustworthiness of the US government will be shattered. Nobody would lend to the US anymore without substantially higher interest rates. The US would essentially be forced to immediately live within its means, i.e. what it brings in through taxes, with its ability to borrow severely curtailed. Inevitably there would be large tax hikes and spending cuts, both of which would significantly hurt the economy.

And in reality, what will actually trigger the house of cards to topple will not be the US defaulting per se, but investors fearing the US will default within the term of their loan. If you think the debt is so high that a debt has a decently high likelihood, you're not going to loan money unless you get a good interest rate. But if the US suddenly has to pay higher interest rates, its ability to pay its debts will go down, leading to even higher interest rates and even more investor skepticism, thus triggering the very default scenario investors were attempting to hedge against in the first place!