For a regression line, squared loss adds the squares of the vertical differences between the observed points and the line’s predictions. A smaller total indicates a closer fit; a larger total indicates a poorer one. section 3.4
Figure 3.19 The square error is the sum of the squares of the vertical distances from the points to the line. Note that the square error is large for the bad model on the left and small for the good model on the right.
Compare the squared distances for a poor-fitting line and a close-fitting line. section 3.4



