Depth of field is important to consider when creating any photograph. It is used at varying degrees to place either everything in the image into a sharp focus or to narrow the focus and highlight a subject, allowing other elements to be blurry.
Depth of field is the amount of your image before and beyond your focus point that will be in focus. You should understand what depth of field is because it will tell you whether or not your subject and background can be sharply focused at the same time.
Photographers use depth of field to create certain effects and draw the viewer's attention to particular elements of the scene. It is important to understand how the aperture setting on your camera, the focal length of the lens, and the distance of your subject affects a photograph's depth of field.
The first couple of photos show a shallow depth of field. The last photo of the staircase shows a medium depth of field. The 2nd row of photos show a deep depth of field.