When the lens is focused on a subject at a given distance, objects within a certain distance in front of and behind this subject are in acceptable focus. The distance, within which the subject is recognized for practical purposes as being in focus, is known as the Depth of Field.
The Depth of Field does not extend to the same length in front and behind the subject. It is narrower in front and broader behind. The shorter the focal length (the wide lenses), the longer to your subject and the smaller the aperture, the greater the depth of field.