The adjustable opening (or f-stop) of a lens determines how much light passes through the lens on its way to the surface of the camera’s imaging sensor. Aperture affects exposure and depth of field of a photo.
Aperture settings are measured in f-stops. The bigger your lens opening (large aperture), the smaller the f-stop number. Vice versa, the smaller your lens opening (small aperture), the bigger the f-stop number.
As you can imagine from this illustration, a large aperture lets in more light than a smaller aperture. A larger aperture also has a shalower depth of field.