When we talk about shooting precision we are referring to the amount of dispersion we expect to see of each shot about a center point (which shooters try to adjust to match the point of aim). Precision is like a cone of error that projects out from the muzzle of the gun. I.e., double the distance and the dispersion also doubles. We can describe this error by referring to dispersion at a specific distance. For example, it is common to quote precision in inches of extreme spread at 100 yards, or "inches per hundred yards."
It is more common, however, to describe the angle of the cone at its tip, since this is independent of the distance at which a target is shot. The higher the precision, the tighter the cone and the smaller the angle at its tip.
One of two popular angular units used by shooters is MOA, though there is some ambiguity in this term. MOA was initially short for Minute of Arc, or arc minute, which is one sixtieth of one degree. At some point shooters began to expand the acronym as Minute of Angle. They also took its original value, which at 100 yards is 3600” tan (1/60 degrees) = 1.047”, and rounded it to 1” at 100 yards. For clarity the latter unit is called “Shooters MOA,” or SMOA.
The other common unit is the “mil,” which simply means thousandth. For example, at 100 yards a mil is 100 yards / 1000 = 3.6”. Some more benign confusion also persists around this term, with some assuming “mil” is short for milliradian, which is another angular unit. Fortunately, a milliradian is almost exactly equal to a mil so there’s no harm interchanging mil, “milrad,” and milliradian.
It is worth noting that 1 mil = 3.6 SMOA \(\approx\) 3.44 MOA.