The holosight has a hologram of crosshairs in it. As you know, a hologram is a 3d photograph. You don't need 3d glasses to get the 3d affect. But you do need the hologrpahic photograph and the light illuminating it aligned in a specific way for it to work.

This gun sight has holographic cross hairs. It's on the glass that you are looking through. This means, no matter where your eye is, the cross hairs you see are aligned with the sight.

With iron sights, you have to put your eye in an exact spot in relation to the rifle to shoot it. Infact, iron sights point is to position your eye in the right spot so that you are looking directly where the gun is pointed.

With the holographic sight, due to the hologram of the cross hairs in it, you don't have to have your eye at exactly the right spot to see the cross hairs. The cross hairs will always point in alignment of the barrel of the gun.