Recently the US has reached a milestone of 30 trillion dollars of outstanding debt. As part of servicing that debt, the US pays interest payments at about 500-600 billion a year. (https://www.treasurydirect.gov/govt/reports/ir/ir_expense.htm)

Since much of the debt is held in treasury bills and bonds, the repayment rate is subject to the federal interest rate. As rates go up, so do things like new mortgages and car loans but also the amount of interest the US has to pay its debt holders.

The yield on a 10 year treasury as of today is 1.91%. If we see significant rate increases by the fed, the $500B in yearly interest payments could balloon to trillions per year. It could print money to pay the interest, but increased monetary supply could result in a vicious inflationary cycle which could mean even higher interest rates.

So in summary: The fed wants to raise rates to fight inflation, but it will have trouble doing so. The US is stuck between the prospect of continued unchecked inflation and the possibility of default by raising rates and increasing repayment obligations.

Here's some more reading on the topic: https://www.cnbc.com/2022/01/27/opinion-the-interest-rate-comet-is-about-to-slam-into-the-us-economy.html