Why does "zeroing" mean for a sight or scope, and what does it do?

I've been finding some low-profile flip-up sights and 4x scopes for my AR15, and I saw a couple reviews say that they "won't zero in" or "will go to have it zeroed". I've been wondering what this specific weapon terminology is. Thanks!
Update: Just in case if it helped, I'm using a 16" barreled AR-15 chambered in 5.56x45mm (.223) My currently-used optics are already calibrated for the .223 cartridge (whatever that means. :P) and are 4x32mm (4x zoom and 32mm lens). I'm currently on thinking about getting a Barska red dot sight soon, but not... show more Just in case if it helped, I'm using a 16" barreled AR-15 chambered in 5.56x45mm (.223) My currently-used optics are already calibrated for the .223 cartridge (whatever that means. :P) and are 4x32mm (4x zoom and 32mm lens). I'm currently on thinking about getting a Barska red dot sight soon, but not until I have "mastered" the ballistics lessons of scopes and optics. ^_^
5 answers 5