A roller rocker is a rocker assembly that uses bearings instead of metal sliding on metal.
A cam/rocker assembly uses a normal cam, but the roller-tip-rocker has a wheel on the end of it like that of a measuring wheel, which rolls by the use of needle roller bearings. Others will add roller bearings for the main pivot also.
The amount of power required to spin a cam is surprisingly high, and it gets harder if stiffer valve springs are used, which are necessary to prevent valve float with aggressive cam profiles at high rpm.