Aperture is the diameter of the lens opening through which light is admitted. When the aperture diameter is increased, then more light reaches the film (traditional cameras) or image sensor (digital cameras).
Aperture is expressed as an F-stop. Here's where it gets confusing:
-The larger the F-stop number, the smaller the lens opening.
-The smaller the F-stop number, the larger the lens opening.
This takes some getting use to, but you need to remember that as the F-stop number increases, the lens opening decreases. Another term to be familiar with is a "fast lens". This refers to a lens with a large maximum aperture (small F-stop number).
A large aperture provides:
-Shallow depth of field (blurred backgrounds).
-Fast shutter speeds.
-Reduces camera shake.
A small aperture provides:
-Large depth of field (foreground and background are all in focus).
-Slow shutter speeds.