A roller rocker is a rocker assembly that uses bearing

Bearing (mechanical)

A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can...

s 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

Power (physics)

In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

 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

Valve float

Valve float is an adverse condition which occurs when the poppet valves on an internal combustion engine valvetrain do not remain in contact with the camshaft lobe during the valve closure phase of the cam lobe profile...

 with aggressive cam profiles at high rpm.