What is Depth of field?
It is the distance between the closest, and farthest objects in an image that would be exceptionally sharp.
Look at the example: This is called background subject separation, the background in this image is blurred to pull the eye to the subject that is sharp. Aperture also controls the amount of light entering the lens, but we will talk about that use of aperture later.
The way to achieve this is by the aperture or f/stop, for example: See the chart below.
The wider the aperture i.e. f/1.2, f/1.4, f/1.8, f/2.8, f/3.5 and so on will produce a blurry background such as what you see in the image of the baby. However, it depends wholly on the widest aperture you can get with your lens. For example, the image of the baby was shot at a f/1.8 aperture, hence the blur in the background or the background subject separation. In the image above the blur is in the background, but it can also be in the foreground, depending on what your subject is, or what you want your subject to be.
Study the chart look at the picture above each f/stop, and you will see blur behind the subject, once you get to f/32 you will notice there is no blur behind the subject. The larger apertures produce the most blur, and the smaller apertures produce the least.