Aperture has two roles. It controls the amount of light that is exposed to the image sensor on your camera and it controls depth of field.
Think of aperture as your eye. The tighter your eyes are closed, the less light you see. The wider your eyes are open, the more light you see. This is what happens when you change the setting of aperture.
The setting of aperture is measured in f-stop which is a number that tells you the distance between the aperture and length of the lens. Don’t understand? Don’t worry f-stop takes some getting used to so don’t feel you have to memorize the definition. Just know that it controls amount of light and depth of field.
Bigger f-stop numbers mean a smaller aperture and less light is reaching the image sensor. Smaller f-stop numbers mean a larger aperture and more light is reaching the image sensor.
The range of standard aperture sizes are as follows:
f/2.8, f/4, f/5.6, f/8, f/11, f/16
These aperture sizes are one stop apart. For instance, f/11 has half as much light as f/16 and so on. Your camera may have more sizes or a setting to allow more sizes. Some cameras will also offer sizes that are measured in one third increments of f-stop.
Depth of Field
In addition to controlling light exposure, aperture settings also change the depth of field. Depth of field measures how much of the image is in focus.
Larger apertures will give the image a shallow depth of field. This technique allows you to blur the background of your image. This is good for close up images.
Smaller apertures will give the image a deeper depth of field and everything will be in focus. This is good for taking scenic images.
Aperture gets easier to work with over time. The key is to practice and cheat. I actually have a cheat sheet (kind of like Cliff Notes) and keep it in my camera bag for reference.