Have you ever taken a shot in Tv or Av mode and had the camera select the wrong exposure? This is quite a common occurrence and is easily tweaked with the the Av button (Aperture/Exposure confirmation button).
Using the Av button we can quickly increase or decrease the exposure. By pressing and holding the Av button and simultaneously turning the dial () we can adjust the exposure between -2 and +2 stops. When in Tv mode we adjust the aperture that camera has selected. When in Av mode we adjust the shutter speed the camera has selected.
To summarize: Regardless of Tv/Av mode, turning the dial to the left (whilst holding the Av button) we can make the picture darker by adjusting the exposure by up to -2 stops in 1/3 increments. If we turn the dial to the right we make the picture lighter by adjusting the exposure by up to +2 stops in 1/3 increments.
This button is very useful! It allows for very quick adjustment of the exposure and it is very quick and simple to use. If you find yourself using the Av button a lot you may wish to use Auto Exposure Bracketing.