"Sighting-in" is a process of adjusting the sights to hit a target at a specific range.
Sight-in instructions are printed on some targets available from retail outlets or manufacturers. Most handguns are sighted-in at 50 feet. The basic steps involve firing at least three shots carefully and consistently at a target. If the bullets form a relatively small group of holes on the target, but not where you were aiming, the sights will have to be adjusted.