It means that it is a currency whose use is imposed on the population by force.  That is the whole story of Bernard Von Nothaus.  He tried to make a currency to compete and the government threw him in jail.  Hence they are making you use the currency by force...  Trying to use a non-federal currency see this article, http://www.newswithviews.com/Ryter/jon120.htm  Again the only reason bitcoin has worked so far is the government is too impotent to stop it, go cryptos!