What is shallow depth of field?

The Basics:

A shallow or small DOF might be used when only a small amount of the image is in sharp focus while blurring any background or surrounding areas in view that could distract the viewer’s attention. To this end, a shallow DOF is particularly well suited to portrait and product photography.

A Little More Detail:

A small or shallow depth of field (DoF) means a smaller range of focus. The closer the focused distance, the less DOF you get - or in more technical terms: the wider the aperture (lower f-number), the shallower the DoF. A shallow DoF means you can deliberately blur out details in either the background or foreground of the scene, allowing you to draw the viewer’s attention to one particular subject or part of your scene. Therefore, a shallow DOF is good for close ups of faces and creating a sense of intimacy around the focal point within your shot.
See Depth of Field and Aperture for more information.