Function of a central bank

The main function of a central bank is to control the nation's money supply (monetary policy), through active duties such as managing interest rates, setting the reserve requirement, and acting as a lender of last resort to the banking sector during times of bank insolvency or financial crisis.

When a central bank is "easing", it triggers an increase in money supply by purchasing government securities on the open market thus increasing available funds for private banks to loan through fractional-reserve banking (the issue of newmoney through loans) and thus the amount of bank reserves and the monetary base. 

Central banks affect the quantity of money in circulation by buying or selling government securities through the process known as open market operations (OMO). When a central bank is looking to increase the quantity of money in circulation, it purchases government securities from commercial banks and institutions.