Technically, MOA (minute of angle) is a measurement of an angle. It equals a width of ABOUT 1 inch at 100 yards. A Red dot that is "5 MOA" would have a dot that on a taget at 100 yards would cover a 5 inch circle. Or a shot group of 2 MOA at 100 yards would be about 2 inches wide.
Now, Dirty Harry said "A man's gotta know his limitations.." There limitations to firearms. A .308 (in the right rifle) can shoot accurately at 900 yards. A .22 LR is running out of steam at over 100 yards, and beyond 150, drops like a bad habit.
Rifles can also be sorted out by type of action (lever, pump, bolt, semi, etc) and purpose (benchrest, plinker, sporter, etc) You would not use a lightweight .22 for serious target shooting- and would not want to CARRY a 12 lb benchrest gun squirrel hunting.