MOA or MILS, Minutes of Angle and Milliradians. These are two methods of doing the very same thing, depending on the type of scope you are using. This is how you site in you rifle and or your handgun.
You know that if you held your gun absolutely level, that when fired the bullet wouldn't go very far before hitting the ground. Well, what we do to correct this is angle the gun upward at a certain angle. This is our MOA and MILS, it is the 'degree' at which we angle our gun to make sure the bullet can reach the desired range, thus hitting the target.