basically a lot of international trade is done using us dollars; i.e. if an aussie company wants to buy some oil from saudi arabia, they will first buy us dollars and then use that to buy the oil from the saudi company. that sort of thing drives up the value of the usd relative to other currencies. the brics are to attempting to bypass that usd trade, using their own currencies directly, or commodity based trading, maybe even a shared trading currency between them.
United States