Another name for a lens opening is diaphragm. Diaphragm is a hole through which the light enters a chamber. This opening or a hole is widely used in cameras and other optical instruments. For instance, the diaphgram opening in a camera affects the amount of light entering the chamber and hence it changes the texture of the final image.
The diaphragm size along with the focal length, change the cone angle and the amount of rays that come to focus in image plane. Hence, knowing how diaphragm affects the final image is considered as one of the many pillars in photography.