A roller rocker is a rocker arm 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 bearings. Others will add roller bearings for the main pivot also. Although roller rockers are used in both pushrod and overhead cam engines, roller rockers for pushrod and overhead cam engines are considerably different in design. For pushrod engines, roller rockers employ a roller where the rocker contacts the valve stem. On the other hand, roller rockers for overhead cam engines employ a roller where the rocker contacts the cam. This greatly reduces frictional losses and wear compared to slipper followers (where the cam simply slides on a surface on the rocker arm). The difference between a roller rocker and a standard rocker with slipper followers on an overhead cam engine is analogous to the difference between roller lifters and flat tappet lifters on a pushrod engine.
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.