Bills are the Treasury's shortest instrument, sold weekly in tenors from four to fifty-two weeks. They pay no coupon; the yield is the discount to face value.
They matter to this paper for one reason: the GENIUS Act permits payment-stablecoin reserves to be held in Treasuries with a remaining maturity of 93 days or less, in repos backed by them, and in money-market funds that hold them. A regulated dollar stablecoin is therefore, by statute, a claim on bills.
When the Treasury Secretary says stablecoins will create a surge in demand for Treasuries, bills are what he means.