Stiff Ring is a washer like ring that is being used to correct the position of the actual washer when it is attached to subframe.

Check out the official release from Stiff Ring team:

Stiff Ring made of special formula aluminium which used in aircraft frame building. It used to correct the car body basic geometry by filling the gap between the sub frame bolts, sub frame and the car body bolt hole. Car engine, transmission, suspension and the body shelf are mounted on the sub frame. Thus, correcting the geometry setup with Stiff Ring, the car handling will be improved significantly.  

From the picture you can see the bolt used to tighten the sub frame and the car body has a gap in between. This will cause the sub frame and body to be miss-aligned and thus lead to a handling problem and produce noise on the uneven road surface.
With the Stiff Ring in proper place, the gap is filled up to stop the movement of the sub frame and the car body forever and provide ideal alignment between both, as a result of a perfect geometry setup.

Speed Mod Stiff Ring is manufactured in Malaysia, thus the cost is reduced compares to other imported brands. We have used special formula alloy aluminium material without compromise to the quality and effect of the Stiff Ring. Our product is strictly controlled under strong Quality Control. There is no doubt to our Stiff Ring quality.

Price for Stiff Ring is about RM220-380.

Check out further explanation by the Stiff Ring itself here: http://www.stiffring.com/