Depth of field refers to the distance between the nearest object (or potion of an object) that appears in focus and the farthest object (or portion of an object) that appears to be in focus. Depending on the distance from the lens, the focal length of the lens, and the aperture setting, the depth of field can be as little as a fraction of an inch, or as great as infinity.
Depth of field
The larger the aperture opening (smaller number) the less depth of field you have. So when you see those great portraits with just the person in focus and the background blurred, that is done by controlling the aperture. When you see landscape photographs with the whole scene amazingly sharp, that is also done by choosing the appropriate aperture.