The original idea was that you could at anytime (practical limitations of course) go and exchange your dollars into a fixed amount of gold and vice versa. With the amount of money in circulation today the entire gold collection of the U.S. treasury is just a drop in the ocean and thus would just provide false security.
Most central banks have therefore reduced their collection of gold, but it has been an "unwritten agreement" between major central banks to do it gradually to avoid a crash in the gold market.
As an example, last year our central bank got rid of all gold:
Gold sale