Federal funds transactions by regulated financial institutions neither increase nor decrease total reserves in the banking system as a whole, instead, they redistribute reserves.[2] Before 2008, this meant that otherwise idle funds could yield a return. (Since 2008, the Fed has paid interest on bank reserves,[3] including excess reserves.) Banks may borrow these funds in order to meet the reserves required to back their deposits. Federal funds are definitive money, meaning that they are available for immediate spending, while checks and many other forms of money must be cleared by banks and typically take several days before becoming available for spending.
The Fed, which is the central bank of the United States, conducts monetary policy primarily by targeting a certain value for the federal funds rate. If the Fed wishes to move to, for example, a more expansionary monetary policy, it conducts open market operations, which include primarily bank reserves; since this puts more liquidity into the banking system, it pushes down the federal funds rate.