The rocker arm that moves the intake and exhaust valves operates on a lever principle. The lever effect makes possible a bigger valve lift than the rotation of the the cam alone can give. But the the more you increase the lever ratio to get more valve lift, the greater the load and friction on the rocker arm where it contacts the cam. To reduce that load and friction a small bearing is fitted to the contact point. This mechanism is called a “roller rocker arm.” This mechanism reduces friction resistance. The resulting reduction in horsepower loss makes for smoother engine revving and also improves fuel economy.