The range at which a rifles sights or optics are zeroed determines the elevation holds necessary on targets of various distances.
Since a bullet is fired on an upward trajectory, its flight path will start below the line of sight by however many inches the bore is offset underneath the optic (2.6 inches for most AR’s). With short zero ranges, the bullet will intersect the line of sight twice; once as it rises and once as it falls. Longer zero ranges will intersect only at the apex of the bullet’s flight path.
For 5.56 platforms:
25/375yrd zero: good for longer range engagements
36/300yrd zero: minimum amount of elevation change for targets 1-300yrds
50/200yrd zero: good for close engagement ranges