Means putting the xhairs on the same plane as the image.
If they aren't the xhairs will move as your eye moves position behind the lens. The parallax knob changes focus of the image so they are same plane. You check it by moving eye position behind the lens for the xhairs to stay on the target in the same spot.
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." — L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...
"I crapped my pants to avoid the draft!!" -Ted Nugent