The opening or aperture of a lens, also known as the iris, greatly affects the amount of light reaching the sensor. The f-number of the lens is the quotient of the focal length of the lens and the diameter of the opening.
For example, a 50 mm lens with a 25 mm aperture would have an f-number of 2.0, as 50/25=2. The higher the f-number, the smaller the opening will be, and vice versa. A lower f-number means that more light will reach the sensor.
Picture: The aperture also affects the depth of field, that is, how much of the scene that can be in focus at the same time.