What does zeroing mean/do?

[Coyote (DayZ) 0]

I read something on the wiki about zeroing on some guns, and today I found a gun that had 300 zeroing, which I could raise and lower. What exactly does this do? I tried to search here on the forums but I couldn't find any explanation of this.

[chrispow 3]

Once fired, a bullet follows a ballistic arc determined by gravity, air resistance, bullet design, etc. Since the muzzle sits below the sights, the weapon's barrel tends to be angled up ever so slightly - this, in turn, causes the bullet to cross the "point of aim" (where the sights are pointing) twice. Once is at close range - less than 50 meters from the weapon, after which the bullet will be slightly above the point-of-aim - while the other happens at what is called the "zero range", which is the range a weapon's sights are calibrated for. After that, the bullet will start to 'drop' below the point of aim. Knowing where to expect the bullet to be at any point along the trajectory helps you to compensate via "offset aiming" for targets that are at ranges other than what your weapon was zeroed for.

http://ttp2.dslyecxi.com/basic_rifleman.html

If your sights are set to 100m and you are trying to hit a target at 300m this will force you to compensate for bullet drop, thus having to aim above the target. If instead you zero your sights to 300m and engage the same target, the aim should be dead on.

[Andrej Dudev 50]

Also, completing Operation Arrowhead tutorials will always help.

[TheGoldElite 0]

Try playing the military simulator you paid for xP. But seriously you coulda have googled it or used the forum search function