US Dollar Index is a measure of US Dollar strength weakness against a basket of six currencies.
These six currencies are the EURO, YEN, POUND, CANADIAN DOLLAR, SWEDISH KRONA, SWISS FRANC.
Surprisingly this basket of currencies does not include Chinese Yuan.
This is despite the fact the China is US biggest trading partner.
So the US Dollar Index is not measuring the true strength weakness of US Dollar right now.
You can read this post in which I explain how you can trade US Dollar Index.
US Dollar Index gives you an idea of the strength weakness of US Dollar.
When you are trading USD pairs like EURUSD, GBPUSD, NZDUSD, USDCAD, USDJPY etc, you shoudl check US Dollar Index also.