Grommets are primarily used to protect wiring being passed through a hole in a chassis or case. They are sort of like a flattened doughnut in shape with a goove around the outer circumference so you can fit them into the hole in the case or chassis. the wiring then contacts only the rubber in the grommet, not the metal that could abraid it's way through the insulation. You can use them to mount items that vibrate, like fans or transformers, with the bolts mounting the component being placed through the center hole of the grommet -- in which case they can isolate some vibration. How effective they will isolate vibration depends on the frequency you are trying to isolate and the stiffness of the grommet. Mounts specifically designed to isolate vibration are often elastomers that are much softer that the rubber used in electrical grommets. Occasionally you will see a component that is mounted only with four blobs of the silcone elastomer similar to what is used to caulk tubs and showers. It stays soft, but adheres well and has good translational strength.