Correlation

What Does Correlation Mean?
In the world of finance, a statistical measure of how two securities move in relation to each other. Correlations are used in advanced portfolio management.

Investopedia explains Correlation
Correlation is computed into what is known as the correlation coefficient, which ranges between -1 and +1. Perfect positive correlation (a correlation co-efficient of +1) implies that as one security moves, either up or down, the other security will move in lockstep, in the same direction. Alternatively, perfect negative correlation means that if one security moves in either direction the security that is perfectly negatively correlated will move by an equal amount in the opposite direction. If the correlation is 0, the movements of the securities are said to have no correlation; they are completely random.

In real life, perfectly correlated securities are rare, rather you will find securities with some degree of correlation.