MOA (Minutes of Angle) and MRAD (Milradian) are two different ways to measure angular adjustments and their relationship to distance.
MOA: Measures in minutes of a degree, with 1 MOA = 1 inch at 100 yards.
Scopes adjust in 1/4 or 1/8 MOA per click, meaning each click changes the point of impact by 1/4 inch or 1/8 inch at 100 yards.
MRAD: Measures in milliradians, with 1 MRAD = 10 cm at 100 meters or about 3.6 inches at 100 yards.
Scope adjust in 0.1 MRAD per click, meaning each click changes the point of impact by 0.36 inches at 100 yards (or 1 cm at 100 meters).