Search the site, there have been many discussions on it before
Basically it is like sticky backed plastic that is cut to shape to bumpers, bonnets etc and it is about 1mm thick. When flies debris hit the front of the car then it kind of absorbs impact without damaging the paint
Obviously large rocks can pierce it but mostly it keeps the front looking brand new
I had an S4 before the RS that did not have it and the front bumper needed repainting because of stone chips, the RS has only one or two
Excellent stuff
Look
here