Charles,
MOA is Minute of Angle. A circle is divided into degrees. Degrees are divided into minutes. So 1MOA is a fraction of a degree. At 100 yards, 1MOA is just barely ( I think 3 decimal places) larger than one inch, so when someone talks about 1MOA accuracy, they mean that at 100 yards the weapon will put the rounds within an inch. You may also hear of rifles with 0.5 MOA which is a half inch group at 100 yards. Red dot or holographic scopes may be listed as having a 3 MOA reticle or dot. This refers to the area on the target that the dot covers at 100 yards.