With the monitor or viewfinder, you may see an image with an aperture that differs from the shooting result. Since the blurring of a subject changes if the aperture is changed, the blurriness of the actual picture will differ from the image you were viewing prior to shooting.
While you press and hold the key to which you assigned the [Aperture Preview] function, the aperture is stepped down to the set aperture value and you can check the blurriness prior to shooting.