Means Minute of Angle. It's a way of measuring the distance between points that doesn't matter what distance they are measured at. I.e., a spread of 1in at 100 yards is the same as a spread of 2in at 200 yards. Thus when people talk about rifle accuracy they just say what multiple of an MOA it can hold so that don't have to say what distance they fired at. For reference, one minute of angle is about 1.3in at 100yds.