A residual is the name given to the difference in response variable value between a datapoint and the value predicted at the corresponding explanatory variable value by the fitted model.

Data points above the fitted line will have a positive residual value and those below the line will have a negative residual value.

A residual plot is similar to a scatterplot with each residual value placed at the same value along the x-axis as the corresponding datapoint, and along the y-axis at its residual value.