The aperture is a part that adjusts the amount of light coming from the lens. As shown in the pictures below, it is located inside the lens, and it adjusts the amount of incoming light by changing the size of the opening.
The amount of light coming into the camera affected by the size of the aperture is quantified as f-numbers. F-numbers have fixed standard values, such as F2, F2.8, F4, F5.6, and F8. As the f-number gets larger, the aperture is closed and less light passes through the lens. As the f-number gets smaller, the aperture is opened and more light passes through the lens. For example, if the aperture is changed from F8 to F5.6, the amount of light is doubled. As a result, even if the shutter speed is doubled, it can let the same amount of light into the camera, as long as other conditions are the same.
The aperture also affects the range of in-focus area, or the amount of defocus in a photograph.
The following shows the comparison between the amount of defocus and the aperture. You can see the foreground and background are defocused more as the f-number gets smaller.
Generally, the smaller the f-number is, the more light comes into the camera, and the more defocus effect you can enjoy. Each lens has a minimum f-number, and this is called the "maximum aperture" of the lens. To check the maximum aperture of a lens, refer to the lens specifications or the value printed on the lens, as shown in the picture below.
A lens with a small f-number is generally called a "fast lens."