Each country has its own currency through which it expresses the value of its products.
In the post-World War II period, the United States agreed to to exchange the dollar at $35 per ounce of gold. The dollar became the common denominator in world trade.
In the early seventies, the U.S. dollar standard was dropped. The result has been more volatility and a more likely tendency for the US currency to depreciate due to persistent US trade deficits.