At its simplest, a Treasury Bill is a promise by the federal government to give the holder $10,000 (or whatever denomination is agreed-upon) in 30 year's time.
In exchange for this promissory note, the holder gives the federal government a certain sum today. What that amount today is depends on how trustworthy the federal government is and the projected inflation rate between now and the maturation of the T-bill.
For
example, if the lender believes that inflation will average (only) 10% per year over the next 30 years, then they would only agree to give the federal government $573.09 today (i.e. pre-inflation) in exchange for being paid $10,00 (post-inflation) in 30 years. Yes, that is right, to raise money today, if 10% will be the average inflation rate, the US government would only get $573.09 for a $10,000 30-year T-bill. And, inflation can easily go over %10/year.
Way over.* The lender has to balance the risk of inflation and the likelihood of eventual repayment on one hand and the amount lent today on the other. The government cannot just offer 30-year T-bills and demand lenders buy them at rates the government would like. The Treasury make them available and has to accept whatever lenders offer.
When the federal government appears likely to repay the principle and the public expects low average inflation, then today's 30-year T-bills will fetch a lot more money todaay. If, however, the federal government is printing money like there's no tomorrow and is even in danger of not paying back the principle at all, the risk premium goes up, and the amount of money lent to get a 30-year T-bill goes down.
The federal government can screw around with this (running huge deficits and just printing money for the difference) a little in the short run, but eventually, the risk starts to creep up and cost of borrowing goes up with it.
When that happens servicing existing debt will crush current federal spending on other things like medicare, medicaid, social security, defense, unemployment, homeland security, national parks, etc. etc.
So go ahead and screw around, Congress, Treasury officials, and Fed bankers. Just go ahead and watch the economic carnage unleashed on the United States. Future generations will curse your memory.
* If anticipated inflation goes up to 20%, then the $10k 30-year T-bill will only fetch
$42 today.