Under normal conditions, the Fed and most other central banks implement policy by setting a target for a short-term interest rate--the overnight federal funds rate in the United States--and enforcing that target by buying and selling securities in open capital markets. When the short-term interest rate hits 5 zero, the central bank can no longer ease policy by lowering its usual interest-rate target.
Because central banks conventionally conduct monetary policy by manipulating the short-term nominal interest rate, some observers have concluded that when that key rate stands at or near zero, the central bank has “run out of ammunition”--that is, it no longer has the power to expand aggregate demand and hence economic activity.