Aperture is one of the most important features when you are setting up your DSLR camera. It effects countless variables of an image. The aperture is an opening in the lens which you can adjust at anytime.
It refers to the access given to the light from the lens to the camera sensors. The bigger the aperture, which means the lower the F-stop number, the more light you let into the lens. To make you understand it better, you can do a little test with your own eyes. Open your eyes completely. After that, almost close them. When you have your eyes completely open, more light will come into your retina. So it was a lot brighter then when you had your eyes almost closed.
Depth of field
As I was saying above, there are countless variables you can adjust with the aperture. For example the depth of field. When the opening in the lens is small, everything in the picture will be clear. When the opening is considerably large, the picture will be blurred. If you set a focus point, this point will remain clear and the background will be blurred. The part of the picture which is clear, is called depth of field. When you have a shallow depth of field it means that a lot of light is let into the lens and most your picture will be blurred. When you have a larger depth of field, it means that not much light is let into your lens and the whole picture will be clear, without blur.
To remember this when you are out and about with your camera. The lower the F-stop, the smaller your depth of field (more light). Likewise, the higher the F-stop, the larger your depth of field (less light).
Your depth of field is most of the time not equally distributed in the front and the back of your focus point. When you increase the focal length, the more equal your depth of field becomes.
You can set your depth of field for every situation. When you change your depth of field it effects all the other settings. So when you change the settings, think about changing the shutter speed. For example, when you are trying to increase your depth of field by reducing the F-stop number (aperture), you will also need to increase your shutter speed which could make your image blurry. To conclude: when you change one variable, it effects everything. All the settings work together.