In its simplest of terms "sub MOA" means that a gun has the ability to shoot slightly smaller than 1" groups at 100 yds.

MOA is 1/60 of one degree of an angle. Extended out to 100 yds. this equals just slightly over 1". At 200 yds. the amount is linear and is just a bit over 2".

Most guns that are described as such on the internet fail miserably to perform at this level in actual use.