Depth of field is the zone of acceptable sharpness within a photo that will appear in focus. In every picture there is a certain area of your image in front of, and behind the subject that will appear in focus.
Shallow depth of field:
Medium depth of field:
Deep depth of field:
More examples of shallow depth of field
This is my best and favorite image because a shallow/meduim depth of field has been used which gives the image that the bench has been stretched out.