[8X56MS]

I am certainly not the guru here, but basically, a 'group' is a cluster of shots, and is used as an indicator of the accuracy of that gun, ammunition, or some other factor.
I think the most common method of measure, is from center to center, of the widest spaced shots. The more shots in each group usually is more indicative of true results. I believe in competition, groups are 10 rounds, but in informal range shooting, a lot of folks brag on their 'three shot groups'
Bear in mind, that the criteria used by one guy to measure his groups, might not be the same criteria used by another guy.
Different scopes, or iron sights, may well have knobs, dials, or whatever, that also differ in adjustment quantity from another brand.