Drawing a line to be followed when cutting. The word "scribing" is not typically used when talking about drawing lines on straight lumber; that can be done simply with a straight-edge. Drawing cutting lines on curved or irregular surfaces, like a log, is a more complicated problem. Scribing is necessary in order to notch logs to mate with other logs, or to cut a flat surface on a log for use as a beam or purlin. You can see one example of a scribed log in this picture, and another one here.