Ordinary Least Squares (OLS)
Ordinary Least Squares (OLS) minimizes the residual sum of squares (RSS). RSS is the sum of each squared residual (residual = the observed Y minus the predicted "on the line" Y).
Also, about the OLS: the average residual is always zero, and the line passes through the point (average X, average Y).
This video by Bionic Turtle explains this method of ordinary least squares (OLS).