Correlation is the calculation of the relationship between two assets.
For instance : The German stock market (DAX) and EUR/JPY showed a similar movement during in the era from 2004 to 2010. During this time, EUR/JPY rose with Positive movement of the DAX performed well and also fell with the negative movement of the stock market .A negative correlation means that asset B will show a opposite movement as. compared to asset a.
This type of correlation can be seen in EUR/USD and dollar index (USDX]. It has been noted that a strong dollar always lifts USDX, but it also increases the dollar against the euro, and that results in a downward movement of EUR/USD.