From Halopedia, the Halo wiki
The iron sight is an open, unmagnified system used to assist the aiming of typically firearms. Multiple Human weapons have a lack of iron sights, but it has been speculated that it is due to the fact that the United Nations Space Command personnel who operate them have HUDs that assist them, negating the need for iron sights. Also, a number of UNSC weapons have concealed iron sights.
Operation and adjustment
Iron sights work by providing and advocating a horizontal and vertical reference point that allow the shooter to actually align the gun parallel to the shooter's line of sight and crosshair. Once the sights are precise with each other, they are placed in correct relation to the target. This places the gun at a precise angle to the line of sight to the target. With appropriate compensation for range to the target, parallax between the iron sights and the gun's bore, and the trajectory of the projectile, a shot fired will hit the target.
Since the eye is only capable of focusing on one plane, and the rear sight, front sight, and target are all in separate planes, only one of those three planes can be in focus at one time. Which plane is in focus depends on the type of sight, and one of the challenges to a shooter is to keep the focus on the correct plane to allow for best sight alignment. A tiny error in sight alignment can be multiplied hundreds or thousands of times by the time the projectile reaches the target.