It's just another unit of angular measurement based on circles.
The MOA brain that actually uses it for field measurements believes there's 360 degrees in a circle, then further brains that into 60 "minutes" of angularity (1 MOA = 1/60th of each one of those 360 degrees).
The mil brain believes there's 6,283.15 mils in the same circle (mil brain calls a circle a radian), then further brains that into a 1,000 equal parts (1/1,000, called a millradian), where .1 mil will subtend .36" at 100 yards, 1 mil will subtend 3.6" at 100 yards, on up to 10 mil subtends 360" at a 1,000.
My brain believes that it doesn't matter to 99% of shooters cuz they'll just use whatever little ruler is in the scope to measure their miss and make an adjustment for a second round centered hit, take a bullet speed, shoot at distance, develop your dope at distance, input it into your rangefinding brain, and let it spit out your dial ups.