when you press it, the lens stops down.
you can evaluate the current depth of field.

it helps you to see how much will be in focus.

is that what the manual said?

The button closes down the aperture inside the lense to the amount that
the camera or settings have defined for proper exposure prior to
capturing the image.

The smaller the aperture the greater the depth of field for the focus.

An example would be to take three photos of a long street. Focus the
camera on a fairly close subject, like a parking meter with the street
running off into the distance in the image. Set the camera on Av mode
for aperture setting. Set the aperture at the smallest number first like
f/4 take the shot, then while still framed and focused take the shot at
f/8, then another one at f/16.

Compare how much is in focus at each setting.

The DOF preview button lets you check how much is in focus through the
viewfinder, but it only works with well lit subjects because the amount
of light that gets through is reduced with each step in the aperture
adjustment. If you notice it gets dark, just keep looking until your eye
adjusts to the reduced brightness level.

Another frustrating aspect is that stopping down the shutter makes
things darker so it's hard to tell. With a wide aperture setting (small
f number) you won't see any difference at all. The lens remains wide
open to get more light until the moment the shutter is pulled and the
DOF preview button simulates this.