In a push-rod Mustang the rocker arm is the lever that transfers movement from the cam lobe to linear movement at the valve to open it. One end of the rocker is raised and lowered by the rotating lobes of the camshaft while the other end acts on the valve stem. When the camshaft lobe raises the outside of the rocker arm, the inside presses down on the valve stem, opening the valve. When the outside of the Mustang rocker arm returns due to camshaft rotation, the inside rises, allowing the valve spring to close.
A roller rocker is a rocker assembly that uses bearings instead of metal sliding on metal.
A Mustang roller rocker has a wheel on the end of it and at the main pivot that rolls by the use of needle roller bearings. The amount of force required to spin a cam is surprisingly high, and it gets more difficult if stiffer valve springs are used, which are necessary to prevent valve float with aggressive cam profiles at high rpm.
A new set of Mustang roller rockers will also allow you to increase the rocker's lift ratio, which will hold the intake valve open longer, thus giving your Mustang more horsepower!