In the drawing included here he aims to practice his depiction of feet, specifically for an upcoming portrait of an apostle. It is likely that he would have done the same of other elements of the scene, perhaps a facial profile or an arm. In some cases he also practiced his use of drapery too, as the way in which cloth hangs from the human body can also be difficult to depict accurately.
The Renaissance, both in Italy and Northern Europe, was full of highly skilled draughtsmen and their work together in the field of drawing makes an excellent aid to those looking to do similar portraits themselves. These artists used a variety of different mediums including chalk, pen, ink and charcoal, with their respective choices being down to a combination of preference and availability with their local region.
This artwork continues Durer's method of adding white heightening to the left hand side of raised elements and darkening the right hand side to produce a shadow and the impression of where the light source is arriving from. It is the detail of the toes which really grabs your interest, appearing so lifelike that one could almost reach out and touch them.