You can buy and sell them just like stocks.

But each one has a maturity date. If you still own it on that date, it goes away and you get your $1000 deposited into your account, which you can use to buy a new one. Or not.

If all you own are the bills, that means that none of them will last a long time before they turn into cash - they aren’t as risky (if you think owning US Treasuries are risky).

A Treasury bill matures in 1 year or less from when it was issued. A Treasury note is more than 1 year but up to and including 10 years. Treasury bonds are anything more than 10 years.