When Changing the size of the Aperture the amount of light entering the lens changes; as the f/stop increases the size of the aperture decreases and the less light enters the lens. As the f/stop decreases the size of the Aperture increases; therefore, the amount of light reaching the sensor increases. But what else does it do? The lens itself acts a lot like an eyeball; the light enters the lens through the aperture and begins to move towards the center of the lens forming a cone. The tip of the cone forms in the middle of the lens and then begins to open up again to form another cone, as shown in the pictures below. (Please excuse the poor drawings lol, I did it myself in Paint.)
The 1st picture represents a lens with a very wide Aperture (very low f/stop) and the 2nd image is the reverse: a very small Aperture (very high f/stop). The little squares inside the lens that have the diagonal lines entering them determine the size of the Depth of Field. When the Aperture is very large (low f/stop) then the boxes (represented in the 1st image) have to be close to each other in order to maintain the path of light traveling within thems. When the Aperture is very small (high f/stop) then the boxes (represented by the 2nd image) have to be far apart in order to capture the amount of light entering.
EXTRA INFO: The reason the boxes change position is because they have to compensate for the size of the cone shaped light entering through the Aperture. The boxes aim to be as full as possible with light; notice how the small boxes have the lines (that represent the light) always close to the outer extremities of the box.
The further apart the boxes are from each other the more in-focused objects will appear in the image at different distances. The area between the boxes is what I refer to as the "Area of DOF" or "DOF field" (kinda redundant lol). The size of this area changes with the Aperture because the Aperture affects the size of the cone of light entering the lens, and the width of that cone of light determines where the boxes that create the "area of DOF" will be located. All of these things are related, and the more you begin to understand that the more your skills as a photographer will develop.
If you have questions, comments, or answers please make sure to leave a comment below or send me an email. I hope this post served it's purpose and that you have a better understanding on the subject. Please be sure to send this to as many people as possible; a couple of photographers I know in different experience levels do not understand this; they might understand how to change Aperture and what effects it has but they don't know WHY it has those effects (in other words, they do not know what is happening inside the lens. Thank you for reading.
Keep on Snappin'