To zero in is to adjust a gun so a target is in its sights. Zeroing in also refers to other types of homing in and targeting.
This term has to do with setting your sights on something, or focusing on something in some other way. If you're zeroing in with a gun, you're adjusting the zero of the gun so it's aimed at a target. Snipers and sharpshooters are experts at zeroing in. If the police are zeroing in on a suspect, they’re getting closer to the suspect. As you adjust a radio dial, you’re zeroing in on a station.