The amount of light reaching the sensor of your EOS depends partly on the brightness of the scene, but is also controlled by the lens aperture.
The aperture is a hole in the centre of a lens diaphragm positioned inside or behind the lens. The diaphragm is usually made up of a series of metal blades that move to alter the size of the hole.
An aperture is the hole in a lens diaphragm. A series of interlocking metal blades move to change the size of the aperture.
Depth of field
The size of the aperture is given as an 'f-number'. This is the focal length of the lens divided by the diameter of the aperture. Large apertures have small f-numbers, such as f/2.0 or f/2.8. Small apertures have large f-numbers, such as f/16 or f/22.
The maximum aperture of a lens is particularly important when you use the viewfinder of your EOS. The image seen through the viewfinder is usually created at the maximum aperture - the larger the aperture, the brighter the image.
In addition to controlling the brightness of the light reaching the sensor, the aperture affects the depth-of-field in the image. Depth-of-field is the area of the scene that appears sharp. A large lens aperture gives a narrow depth-of-field - only a small area in front of and behind the focused distance appears sharp (f/1.8 in the animation below).