Answer:
The word aperture refers to the diaphragm opening inside a photographic lens. it works in a similar way to the iris in the human eye. Just like the iris regulates the amount of light that passes into the eye by closing and opening, the size of the diaphragm opening in a camera lens regulates the amount of light that passes through the lens onto the film inside the camera at the moment when the shutter curtain in camera opens during an exposure process.
The size of an aperture in a lens can either be fixed (like most compact cameras) or adjustable (like an SLR camera). Aperture size is usually calibrated in f-numbers or f-stops, i.e. those little numbers engraved on the lens barrel, e.g. f/22, f/16, f/11, f/8.0, f/5.6, f/4.0, f/2.8, f/2.0, f/1.8 etc. Each of these values represents a factor of the amount of light that passes through the lens. The smaller the number, the larger the aperture, e.g. a standard f/1.8 50mm lens has a largest aperture of 1.8. Each following f-stop lets in half as much light as the previous, so f/2 will let in half as much as f/1.8, f/2.8 will let in a quarter as much, etc. The result of this is that the exposure time is doubled each time a smaller aperture is selected.
The aperture size will also affect the field of depth, so that a portrait photographer will normally choose large apertures (small f-stops) which produce very small depths of field (subject in focus, background out of focus) and a landscape photographer will choose small apertures (large f-stops) which produce large depths of field (everything in focus from near to far)