An aperture is a hole or an opening through which light travels. The aperture of an optical system is the opening that determines the cone angle of the bundle of rays that comes to a focus in the image plane. The aperture determines how collimated the admitted rays are. This is of great importance for the appearance at the image plane. If an aperture is narrow, then highly collimated raps are admitted. This results in a sharp focus at the image plane. If the aperture is wide then un-collimated rays re admitted. This results in a sharp focus only for rays with a certain focal length. The aperture also determines how many of the incoming rays are actually admitted and thus how much light reaches the image plane.
Optical systems typically have many openings that limit the ray bundles. These structures may be the edge of a lens, mirror, ring or other fixture that holds an optical element I place. These structures are called stops, and the aperture stop is the stop that determines the ray cone angle or equivalently the brightness at an image point.
Aperture stops are important elements in most optical designs. In more obvious features being that it limits the amount of light that can reach the image. The size of the aperture also affects the depth of the field, limits the effects of optical aberration and determines whether the image will be vignette. An aperture stop in a photographic lens may have one or more field stops.