In this mode you set the aperture and the camera sets the shutter speed automatically to suit the brightness of the subject
A large aperture (small f-number) will blur the background and make the subject stand out. The larger the aperture the more blurred the background.
Or a small aperture (large f-number) will increase the depth of field to make the foreground and background look sharp. The smaller the aperture, the sharper the background will look
*"Av" stands for Aperture Value