Correlation refers to the relationship between the values of two separate assets. Positive correlation means that the prices of the two asssets move together in the same direction. Negative correlation means that the prices move in opposite directions. The price of something that is uncorrelated does not make any measurable reaction to the price of the thing that it is uncorrelated with.
Correlation, low Correlation, un-correlation or negative correlation are an important factor in multi-asset investing and diversification.