Aperture is a strange word for the size of your shutter hole while its taking a photo. The larger the opening, the more light will be captured. On the other hand, the smaller the hole the less light goes in. Once again, we see a setting that is just another way to toggle the brightness of your image. But while changing the shutter speed in manual mode leads to motion blur, changing the aperture size on your camera will have a different host of side effects.
The best way to understand is to try it yourself. Open your eyes as wide as you can and stare at one specific object that you can see. As you force your eyes wider and wider, and your focus almost becomes eyed, you will see the object that you’re focusing on become brighter and sharper. But notice that everything else around you, especially things in the background behind your focal point, seems to lose focus. The wider your eyes get as you stare at one point, the less you notice of the surrounding objects. This is the exact effect of a small aperture value, which confusingly, represents a larger shutter opening. (Yes, just to make it harder to remember.)
If you squint your eyes now, and still focus on the same spot, you will notice that the focal object is now a little hazier, but everything else is more in focus, or at least harder to tune out and not notice. This is the effect of setting a larger aperture number, which makes the lens opening smaller.
How to use it
How does this get applied in photography? Most noticeable, aperture is the setting that many portrait artists use to create those blurred backgrounds behind the subject. In these shots, the aperture may be something like f/2.8, which is very wide.
Be careful at these low apertures though, because it would easy to add too much light to the photo, which would result in a blown-out image, where the light overwhelms everything else in the photo.
On the other hand, a landscape shot where everything must be clear will use a small aperture like f/22. At these high apertures, you may have to adjust other settings like shutter speed and ISO just to get enough light.