Basically they are angle measurements.
MRAD = MIL (aka Milliradians) -- that's the military/metric standard (6400 mils in 1 complete turn). It's also defined as 1 mil = 1/1000 -- i.e. 1yard @ 1000 yard distance, or 0.1 yds (3.6") @ 100yds, 0.5m (19.68") @ 500m
MOA = 1" at 100 yds.
e.g.
A 6-ft man at 100 yards = 20 mils (or 20 MRAD) tall as seen by an MRAD/Mil scope reticle
Same 6-ft man at 100 yards = 72 MOA tall as seen by an MOA scope reticle
BTW, if the scope is NOT a first focal plane scope, you have to set it to the correct zoom factor before estimating ranges. This is usually max zoom or 12x zoom on some scopes. In a first focal plane (FFP) scope, the reticle zooms with the image and stays calibrated so you can estimate ranges at all magnification factors.