Aim of small residuals
The residuals from a linear model (vertical distances from the crosses to the line) indicate how closely the model's predictions match the actual responses in the data.
'Good' values for b0 and b1 can be objectively chosen to be the values that minimise the residual sum of squares. This is the method of least squares and the values of b0 and b1 are called least squares estimates.
The diagram below respresents the squared residuals as blue squares. The least squares estimates minimise the total blue area.