What is a roller valve spring?

Roller rocker arm. The rocker are in the head that transfers the camshaft action to the valve. Opens and closes the valves. On a roller rocker, there is a small roller on the lifter end of the rocker arm. Some small gain in performance due to lack of friction. Very small. In a complete engine overhall where everything is done just right to improve performance, each little bit can have a combined affect that does improve the overall performance. Changing the rockers by themselves is not wort the expense.