The dollar index is an indication of the strength of the USD in relation to some of its most significant trading partners. In fact there are 6 major world currencies that come into play with the Dollar Index (DXY). They are the Euro, Japanese Yen, Canadian Dollar, British Pound, Swedish Krona and the Swiss Franc.
When created this index started with a base point of 100. This in turn now shows relative appreciation or depreciation of the US dollar in terms of how many points below or above this base point the index is.
So if you saw a price of 80 this would mean that the relative value of the dollar has decreased 20% since the launch of the DXY.