milliradians = MRAD = MILS
MOA is Minute of Angle, and is based on the traditional 360 degree measurment of a circle (a single degree broken down to minutes and seconds) however, for the purposes of scopes etc., minutes are divided into quarters.
Milliradian is 1/1000 of a radian. You can go down geometry rabbit hole to learn more about how many radian are in a circle if you like. For the practical application here, it is 1 meter at 1000 meters and is divided down by decimal. Effectivly metric, but only if you are ranging in meters. But that doesn't really matter.
The short answer is mils is a decimal based system vs MOA which is broken into quarters. In my opinion, the math with mils is far easier.
1/4 MOA is approx .25 inch at 100 yards and .1 mil is approxl .36 inch at 100 yards. So MOA adjustments are more refined, but in practical precision rifle applications, not a material benifit.