The coefficient of determination, $r^2$, is the square of the correlation coefficient, and describes the degree to which one variable can be predicted from another.

If $r$ = 0.8, then $r^2$ = 0.64 or 64%. This indicates that 64% of the variation in y (the dependent variable) can be explained by the variation in x (the independent variable). This means that 36% of the  variation in the dependent variable will be explained by other factors other than the independent variable.