Aperture is the opening of a lens on your camera that allows how much light goes though. The aperture refers to the size of this iris (hole) within the lens. The bigger the hole, the more light that enters the camera. Aperture is measured in F-stops, this determines how wide or small the aperture will be. It is specified in terms of an f-stop value, which can at times be counter intuitive, because the area of the opening increases as the f-stop decreases. In photographer slang, when someone says they are "stopping down" or "opening up" their lens, they are referring to increasing and decreasing the f-stop value, respectively. A low F-stop such as f1.8 means a wider aperture, however a large F-stop such as f16 will allow the image to be a smaller aperture.
Iman Khan