Aperture refers to the opening, which is actually located in your lens,
that allows light to enter your camera. To understand how it works,
imagine that you are in a darkened room with a window covered by
curtains. If you open the curtains, light enters the room. The wider you
open the curtains the more light enters the room. If you close the curtains
the room becomes dark again. You are controlling the amount of light entering
the room the same way you adjust your aperture to control the light
entering your camera.

If you want to know the size of the window you measure it in feet and inches,
or meters. A four foot window is smaller than an eight foot window.You measure
your aperture in fstops. Here's where many people get confused with fstops - the
smaller the number the larger the aperture.
So f4 is a larger opening than f8.
Here are some fstops you will encounter from largest to smallest- f2.8,f4,f5.6,f8,f11,