MOA means minute of angle which is 1/60th of 1 degree.
Before calculators were available to convert things into decimals easily degrees were commonly broken up into minutes, then seconds for precise measurements.
With regards to firearms it's a reference to the accuracy. If a rifle is accurate to one minute of angle it means that the trajectory of every bullet leaving the barrel will be within 1/60th of 1 degree of each other. That translates to roughly a 1" group at 100 yards.
You can use a calculator pretty easily to show this.
100yds = 3600in, so 3600*tan(1/60) = 1.047
In a giant clock the minutes would still each represent 6 degrees which would give you a group of 3600*tan(6) = 378.37" or about 30 feet.
I actually use that question as an example in class when we cover trig functions. And you thought you were never going to use Geometry after High school.
