What does aperture mean?
Aperture is the setting on a camera that controls how much light passes through the lens to strike the camera sensor (or film), thus it serves as a brightness control. Aperture is expressed in f-stops such as f1.2, f8, f22, etc.
How does aperture effect a photo?
Increasing the number of the aperture decreases the size of the opening, and therefore decreases the amount of light that it allows to pass through the lens.
Another effect of aperture is that the smaller the opening (the larger the aperture number) the larger the distance that is in focus beyond and in front of the focus point (this distance is called depth of field). This effect can be used to create a sharp focus on the subject and blur in the rest of the photo, or it can be used to keep the entire image in focus. The image above is an example of using aperture to set the depth of field, and focus on the subject but not the background.