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It means that they use their own currency to buy oil. Most countries have to convert their currency into US dollars to buy oil. If China starts using their own currency, they will greatly decrease the demand of the US dollar and the supply/demand of the US dollar which shift. The glut of supply of dollars will have a drop in price (foreign exchange).

If global demand for USD collapses like that, all the USD (inflation) the US has been exporting since the 70s will come back home, most likely leading quite rapidly to hyperinflation.