Absolute error measures how closely a line fits a dataset. For each point, take the difference between its observed label and predicted label, make that difference positive with an absolute value, then add the distances. A smaller total indicates a better-fitting model. section 3.4
Figure 3.18 The absolute error is the sum of the vertical distances from the points to the line. Note that the absolute error is large for the bad model on the left and small for the good model on the right.
Compare the bad and good fits: the figure shows how their summed vertical distances correspond to larger and smaller absolute errors. section 3.4

