Back in the good old days, many currencies including the USA had their money backed by gold and silver. You could take that $5 silver certificate to a central bank and would receive $5 in silver if you so desired. (Probably around 2 ounces back then) After 1971, the gold/silver standard was officially replaced with an entirely fiat based currency. Our money now isn't backed by anything but the promise from the Fed that it's valuable. As long as us citizens accept it as valuable, we should be able to continue buying goods with it.