its hard rubber pieces attatched to he top of the strut, usually five or four layers and you cut two off. this allows the strut to compress in even more giving it a stiffer ride: you want the strut to be as stiff as possible when lowered so that the car wont bottom out and cause damage. i think it all depends on how low you are going too, if your only going 1-2 or similar i would'nt worry to much about. With coilovers I think most manufacters recommend it. I did it and it was easy enough.