The aperature is the hole through which light enters a camera, increaisng the size allows more light in, thus improving vision in low-light conditons, but redcues the sharpness of the image. Crucially a smaller aperture alows a graeter depth of field, that a great range of objects are in sharp focus. One therefore usually aims for the smallest aperture that is usable given the light conditions.
Used on Chap. 15: page 362