Photography is all about understanding and manipulating light. The understanding of light and the ability to use the camera controls to capture your images manually proves to be a fundamental part of photography.
The aperture is arguably the single most important control of any camera as it controls the volume of light entering the camera via the lens.
Essentially, an aperture is a hole that you can control the diameter of, the smaller you make the hole the less light enters the camera and therefore your image exposure decreases, and vice versa.
So you can control the exposure, but 2 other settings offer a similar feature, that of the shutter speed and ISO. However, the aperture also controls the depth in the plane of sharp focus with a result called depth of field that describes the amount of the image that appears sharp to the viewer in front of and behind the focus point. If you were to shoot on a wide aperture such as f2.8, the zone of sharp focus or depth of field will be reduced, and if you were to shoot on a small aperture such as f22, the zone of sharp focus or depth of field will appear to be greater to the viewer.
Take a look at the video below by the YouTube channel: Apalapse on Camera Basics - Aperture...