Depth of Field
As stated, aperture controls depth of field. Depth of field is the distance, both in front of and behind the plane of sharp focus, where objects remain in reasonable focus. In other words, how much of the image is in focus from near to far. Depth of field is controlled by three factors: aperture, the focal length of the lens, and distance to subject.
For a given lens, the smaller the aperture (the bigger the f/number), the more depth of field.
The shorter the focal length of a lens, the more depth of field at a given aperture and distance.
Distance to Subject
The closer to the subject, the less depth of field with a given lens focal length and a given aperture.