The image on the monitor or viewfinder may have an aperture value different from that of the actual image to be shot. 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.