Whenever a photograph is taken, a decision has to be made about how much light is needed to achieve an acceptable exposure. Often the camera will automatically select a shutter speed, aperture and ISO to control this. However, the photographer may decide to select one or more of these variables themselves.
The aperture of a lens refers to the size of the opening through which light passes. A certain lens will have a maximum aperture (the size when it is as wide open as possible) but can be set to close down to a narrower size.
If two photographs are taken under the same conditions, using the same shutter speed and ISO but with different apertures, then the photograph with the smaller aperture will appear darker. A small aperture lets in less light than a large aperture.
The progression of apertures (in “stops”) is as follows: f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32… Closing down the aperture by one stop halves the amount of light reaching the sensor, so the shutter has to stay open for twice as long to let in the same amount of light.
(diagram from Wikimedia Commons)
The photographs below demonstrate the effects of changing the aperture. Not all lenses can achieve every possible f stop. For the experiments shown below, I used a lens whose widest aperture was f2.8.