Correlation assesses the association between two variables. There can be many types of correlation measures. A common one is the Pearson correlation coefficient that assesses the “linear” association between two continuous variables.
A Pearson correlation coefficient lies between -1 and 1. A positive coefficient means a positive linear relationship (i.e., both variables move in the same direction), while a negative coefficient means a negative linear relationship (i.e., the two variables move in opposite directions). Both -1 and 1 refer to perfect linear relationship while 0 refers to no linear relationship.