The value of money is mainly tied to the inflation rate prevalent at that time. The federal reserve's job is to prevent disinflation and maintain inflation at a moderate rate (2-3%). They are able to accomplish this through monetary policies such as controlling the LIBOR rate which affects short term interest rates between banks, which in turn should affect short-term interest rates everywhere. They also accomplish this by buying and selling bonds in the open market to increase and decrease the money supply in order to spur the economy or slow down the economy.
