THE APERTURE is built inside each lens and it controls the light entering a camera.  A lot like the shades on a window control the light:

“Stopped down” f16 or f22

Measuring Aperture

Aperture is measured in f stops. The larger the number under the f, the smaller the hole (aperture) and the less light that comes in.

A wide open aperture results in a shallow depth of field whereas a smaller aperture results in a great depth of field.