What is Sub-MOA?

A MOA (one minute of angle, means 1/60 of a 1 degree) is unit used in long distance shooting.

You can use it also to describe precision of a rifle: if a rifle is sub-moa precise, you can say that shooting the target where you aim at the same point all the time, all the holes will be in a circle that has size less then 1 MOA.

Example:
you shoot at 100 yards - all the holes will be in a 1 inch circle
200 yards - 2 inches
300 yards - 3 inches