The aperture is the hole through which light enters a camera, increasing the size allows more light in, thus improving vision in low-light conditons, but reduces the sharpness of the image. Crucially a smaller aperture alows a greater depth of field, that is a greater range of objects are in sharp focus. One therefore usually aims for the smallest aperture that is usable given the light conditions.
Used in Chap. 15: page 233