Micro-Groove rifling is a Marlin
registered and
patented rifling method which gives a better gas seal and less bullet deformation which gives you better velocity and accuracy.
It is micro in the sense that the lands and grooves are markedly narrower on these than on conventionally* rifled barrels. Even in small .22 caliber bores, they can fit up to sixteen grooves. Compare this to conventional rifling of big bore pistol or rifle barrels which usually only have 4 to 8 grooves.
*
see ballard cut rifling, button rifling, lands and grooves, and also polygonal rifling for more enlightenment.