Earlier quoted context omitted.
I described Open Market Operations, see https://en.wikipedia.org/wiki/Open_market_operation > The Fed sets the interest rate directly. So which interest rate does the Fed set directly, and how does that setting have any effect on the economy? (I know they have interest on excess reserves. I already accounted for those in my original comment. I know, they are annoying and misguided. I was mostly talking about the syst…
The Fed directly sets the Overnight Rate, also known as the Federal Funds Rate. It's the rate at which banks can borrow from one another overnight to satisfy reserve requirements. This rate indirectly affects the interest rate of all other lending instruments because the higher cost of overnight bank to bank lending is passed on to the customers in the form of higher loan rates, credit card rates, mortgage rates, etc…
As a counterfactual: we can imagine a world where the Fed 'sets' these rates by just announcing the rate but not engaging in any transactions and legislators pass a law that forbids banks from borrowing from one another at any other rates.
The interbank lending market would just not clear in that case, and you'd either have a glut of demand or a glut of supply.
The Fed sidesteps this by actually borrowing and lending in the interbank lending market. And that's economically equivalent to buying and selling very short term bonds, even if the legal form is different.