"Av" is a camera stetting which stands for "Aperture priority". This tells the camera what aperture you want, and the camera will decide the shutter speed on its own. You select Av if controlling the aperture is the most importiant aspect of the photo. If shutter speed is the most importiant aspect, Av can still be used, as the lowest aperture value i.e. f/1 will deliver the highest possible shutter speed. Selecting the highest aperture i.e. f/32 will force the slowest shutter speed while properly exposing the image in the eyes of the camera's selected metering system. Of course what the camera thinks is right and what you think is right may clash. In this case you use the "exposure compensation" dial to make the image "darker or lighter" by increasing or decreasing the camera's recommended shutter speed.
Aperture, Shutter Speed, and ISO are all required for a photograph. Every camera in existence uses all three. Film cameras don't use "ISO", instead opting for "ASA". ISO and ASA are exactly like an analog watch and a digital watch. They both tell time but have different ways of going about it. For all practical purposes, they are the same thing. If you take any 1 of these three elements out of a photograph, there will be no photograph. From the last statement, each one obviously has an effect on the other two. Understanding what what each does, why they do it, and how they do it is a very importiant first step of understanding photography. A 4th less critical element would be flash. A 5th element would be image stabilization or a tripod. You can't possibly understand how to properly use flash, image stabilization, or a tripod without first understanding Aperture, Shutter Speed, and ISO.
