MOA stands for minute of angle. that is one inch per every 100 yards. So if someone shoots 1MOA that means at any given distance (for example 200 yards=2inch groups) the shooter is getting groups of paralell size. which is not always the case so usually you need to specify at what distance. it's used for groups, scopes (eg. 1 click =1/4 inch at 100 yards on most hunting scopes).