The Senate voted overwhelmingly and in a bipartisan manner to threaten China over its exchange rate policy. Supporters of the threat argue that an undervalued yuan (meaning an overvalued dollar) is a problem for the U.S. economy because it gives China an advantage in trading with the U.S.
This is truish, but beside the point. An artificially weak yuan is a huge advantage for U.S. firms and consumers who buy goods from China. Arguing against that is the same as threatening your grocery store with sanctions if it doesn't raise its prices.
Dave Nicklaus has a good wrap up of the trade-war side of the argument.