THE WHAT= The USDX or DX-Y.NYB common term 'the dixie' is the US dollar index future (DXY). It is a index of the US dollar vs the rest of the major pairs in the exchange lumped into a basket. As a forex currency pair it would look like this-----USD/(EUR,CHF,JPY,GBP,CAD,SEK). You can trade the DXY on the futures market.
THE HOW= This is trickier, obviously if the DXY is bullish then the USD is showing strenght against all other major currency pairs. But if its bearish it could be a few different things or a combo of them. Either the USD is weakening solely itself or any number, combinations or all of the other currencies are strengthening against the US dollar. Also the CAD is closely correlated with crude price behavior and this tends to be a factor also occationally. So as a indicator for the chart many traders like to keep an eye on the DXY to create or affirm a trade bias they may have.