In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.

An optical system typically has many openings or structures that limit the ray bundles (ray bundles are also known as pencils of light). These structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place, or may be a special element such as a diaphragm placed in the optical path to limit the light admitted by the system. In general, these structures are called stops, and the aperture stop is the stop that primarily determines the ray cone angle and brightness at the image point.

In some contexts, especially in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself.

The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. In combination with variation of shutter speed, the aperture size will regulate the film’s or image sensor’s degree of exposure to light. Typically, a fast shutter will require a larger aperture to ensure sufficient light exposure, and a slow shutter will require a smaller aperture to avoid excessive exposure.

In easy words, aperture is a hole within a lens, through which light travels into the camera body. It is an easy concept to understand if you just think about how your eyes work.

This is also known as F value / F numbers of your camera. You need to remember that a large aperture results in a large amount of background blur. This is often needed for portraits or photos with blurry background.

On the other hand, a small aperture results in a small amount of background blur which is typically ideal for landscape or architectural image.

Aperture / F Values list:

f/1.4 Very large
f/2.0 Large
f/2.8 Large
f/4.0 Moderate
f/5.6 Moderate
f/8.0 Moderate
f/11.0 Small
f/16.0 Small
f/22.0 Very small