The aperture is made up of a set of expandable and contractable metal blades in the camera’s lens that form a hole. This hole is responsible for letting light onto the camera’s sensor. Opening that hole up wide lets the most amount of light onto the sensor while closing the hole down lets the least amount of light onto the sensor.

F/Stops are a ratio of two things: the diameter of the aperture in the lens and the focal length of the lens. For instance, if we had a lens at a 135mm focal length, adjusting the aperture to F/3.5 would mean the aperture (hole) is open 38.6mm (135/3.5 = 38.6). A 135mm lens at F/22 would mean that the aperture is open 6.1mm (135/22 = 6.1). Therefore, the higher the F/Stop (aperture value) the smaller the hole, while the lower the F/Stop the wider the hole.

Courtesy: TechnoBuffalo

Traditional apertures double or halve the light. As an example, F/2.0 is twice as bright as F/2.8. F/11 is twice as bright as F/16. The following sequence is the standard for aperture values:

F/1.4   F/ 2.0   F/ 2.8   F/ 4   F/ 5.6   F/ 8  F/ 11   F/ 16   F/ 22

As you look at the above F/Stop sequence, think wide –> narrow. An F/1.4 lens is regarded as a very bright or fast lens while an F/22 lens is regarded as a slow lens. 

Note: The aperture is the size of the hole that light passes through to hit the sensor. The shutter speed is the amount of time the aperture stays open.