Original sentence: Here the “best” will be understood as in the least-squares approach: such a line that minimizes the sum of squared residuals of the linear model
Edited sentence: (Note that a straight line may not be the appropriate regression curve for the given data points.) Here the “best” will be understood as in the least-squares approach: such a line that minimizes the sum of squared residuals of the linear regression model.In other words, if linear regression is appropriately applied to a set of data points  whose sample correlation coefficient  is non-negative, and not 1, then there is regression toward the mean