The slope of the best fit linear regression is a measure of the rate of change of the regression of one independent variable on the dependent variable.

The plot of the linear regression on the variables is used to predict the behavior of the dependent variable from the change in the independent variable.

Note that this computation assumes a linear relationship between the variables.

There can be a strong nonlinear relationship between independent and dependent variables, and the computation of the simple linear regression between such variable pairs does not reflect such a relationship.