In photography, aperture refers to the size of the opening in the lens of the camera through which light can pass. The size of this aperture is adjustable in nearly all the lenses that fit digital cameras. By adjusting the size of the aperture, the photographer can ensure that the correct amount of light reaches the digital sensor during any given exposure.
As such, it is one of the three elements used to provide a correctly exposed image. The others are the length of the exposure, called the shutter speed, and the light sensitivity of the sensor, called the ISO.
The aperture can be adjusted either manually or, in most cameras, automatically by the camera. When the diameter of the aperture is changed, a set of blades inside the lens narrow down or open up to allow more or less light to pass through the lens.
The act of narrowing down the aperture is often referred to as 'stopping down' while opening it up is called 'stopping up'.
Novices often find the word 'stop' connfusing, but it is easier understood when one realises that the aperture is, by convention, adjusted not over a continuum, but rather is set steps, or stops. One stop is equal to half the area of the one preceding it. So when you close down one stop, you are halving the area of the aperture, and when you open up by one stop, you are doubling it.
The stop selection dial (which selects the size of the aperture) is marked with an odd set of numbers as noted above, often starting at f5.6, followed by f8, then f11, f16 and f22. There may be one or two stops on either end of the scale. The concept works like this. The number indicates the inverted size of the aperture as it relates to the focal length of the lens. This is quite a mouthful, and not easily understood, but the point is that the larger the number, the smaller the hole, and therefore, the less light is allowed to pass through the lens. The smaller the number, the larger the hole, and the more light is allowed to be transmitted. So, other things being equal, the brighter the light in which the picture is taken, the less light would be needed for an accurate exposure and the bigger the f-number one must use. Inversely, the dimmer the light, the bigger the hole that is needed, and the smaller the f-number that is selected. Once you understand this, you are halfway to grasping all that is needed to become a master of the aperture.
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