Federally-insured funds. They can borrow money directly
from the Fed using the "discount" rate, or they can borrow from each other using the "federal funds" interest rate. Both are short-term or overnight rates.
The discount rate is designed to improve liquidity for the banks themselves. The federal funds target rate rate is meant to impact consumer credit.
Because the Fed can't dictate what happens in the market, the Fed will issue a "target" rate for federal funds, which most banks stick close to. They can then charge consumers whatever they feel they can get away with in the form of credit card interest rates, mortgage interest rates, car loans and so on.
One thing the Fed does not do is set mortgage interest rates. The lenders do that based on such factors such as the yields from mortgage-backed securities. When the bond yields go down, interest rates follow. Bond yields go down when the market believes inflation is under control.
If that's true, mortgage interest rates will drop, and housing becomes more affordable.