Aperture or depth of field is the amount of distance between the nearest and farthest objects that appear in focus in a photograph. In simpler term, we define depth of field as the zone of sharpest focus in front of, behind, and around the subject on which, when lens is focused on a specific subject.
There are three key factors that have a direct relationship with depth of field, aperture, focal length of the lens, Subject size.
The general rule for selecting the right aperture for a desired depth of field is: given the same distance to the subject and subject size, the bigger lens opening (aperture) used (like f/2.8, f/2, f/1.4 etc.) will have a narrower band of depth of field.
Depth of field increases with distance. The farther you place the camera from your subject, the more depth of field you can obtain. Landscapes have great depth of field, while macro photographs tend to have very little depth of field because the subject is so close to the lens.
Take look at my video