What is shading?
Shading means darkening of certain areas in a sketch or an image by application of a darker medium to enhance the part.
In simple terms shading is trying to darken a part of image or sketch by applying constant strokes on the area with a flat application or pointed application of the pencil.
The exertion of pressure on the pencil gives the image varied shading. Darkening an image with great pressure exerted by the hand will give out darker image than when pressure is applied lightly.
Shading always produces optimum results when applied in one direction only to avoid patchiness and is done by the side of the pencil.
This technique of giving depth to an object is the reason why 3-dimensional object portray their dimensional form and a sketched human face seems so alluring.