Aperture, in a mechanical sense, refers to the size of the opening at the back of the lens, which controls how much light is being passed onto the camera's sensor.
Aperture, often called F-stop, is important to the exposure of your photograph, but it also affects how much of your image is in focus, or the 'depth of field'. This is an important stylistic choice, and impacts the look and feel of your image.
copied from internet