MOA means Minute of Angle
Youtube search Intro to Minute of Angle if its the video I watched a ex sniper trainer explains it pretty good. Basically it's 1" @ 100 yards / 2" @ 200 yards which lets you calculate how to hit dead on at other yardage. (Rough explaination.)
As far as a stock for the M&P 15t I'm not farmiliar with them but I would guess that any stock matching your tube size would work fine, IE Mil Spec or Commercial... being a smith and wesson I would bet its Mil Spec.