A minute of angle is a slice of a circle: one sixtieth of one degree. That sounds useless until you put a distance on it. At 100 yards, one MOA covers just over an inch, 1.047 to be exact, and it grows with distance, so it is about two inches at 200 and about five at 500. For almost everything below, “an inch at 100 yards” is close enough.
Accuracy claims. A rifle called “sub-MOA” is supposed to put a group of shots inside one inch at 100 yards. A half-MOA group is about half an inch. Treat the number as what the rifle can do with good ammunition and a shooter having a good day, not what it will do off the tailgate.
Scope clicks. Most scopes adjust in quarter-MOA clicks, so each click moves your point of impact about a quarter inch at 100 yards. If you are four inches low at 100, that is sixteen clicks up. At 200 yards the same click is worth half an inch.
Red dot size. The dot itself is measured in MOA. A 2 MOA dot covers about two inches of target at 100 yards, which is fine for precise shots. Larger dots, and the big circle reticles, are faster to pick up up close and cover more of the target farther out.
Some scopes use milliradians, mils for short, instead. A mil is about 3.6 inches at 100 yards, and mil scopes usually click in tenths, about a third of an inch each. Neither system is better. What matters is that the reticle and the turrets use the same one, so the marks you see and the clicks you dial speak the same language.
We keep red dots and LPVOs on the shelf and mount pistol optics and red dots at the counter, free when you buy the optic here (pricing). If you are choosing a dot size, look through a few first. It is also the quickest way to find out whether astigmatism turns a crisp dot into a starburst for you, which a spec sheet will never tell you.