Aperture refers to the size of the hole in which light travels through to get to the camera sensor. The larger the hole the more light is allowed in. The Aperture is a function of the lens rather than the camera body. The size of the hole in specific lenses is referred to as the F number, and it is measures in stops which is the halfing or doubling of light absorbed by the cameras sensor.  The lower the F number the larger the hole in the lens.