A rubber grommet is a ring or edge strip inserted into a hole through thin material, flared or collared on each side to keep them in place. A liner that goes into the hole and a cap with a hole (often adjustable in size) for the cables to go through.
They may be used to prevent tearing or abrasion of the pierced material or protection from abrasion of the insulation on the wire, cable, line being routed through the penetration, and to cover sharp edges of the piercing, or all of the above.
A rubber grommet is made of a resilient material (typically rubber), with the molding designed to hold it in place, so as to help to absorb vibrations, for example, between a tandem radio and the chassis or between microphone and its tripod, keeping the two components “floating” mechanically decoupled one from another to prevent a characteristic coupling called microphonism .
If metal or another hard material has a hole made in it, the hole will probably have sharp edges. Electrical wires, cord, rope, lacings, or other soft vulnerable material passing through the hole can become abraded or cut, or electrical insulation may break due to repeated flexing at the exit point. Rubber, plastic or plastic coated metal grommets are used to avoid this. The grommet could also protect the wiring/cabling from contamination from dirt, air, water, etc. The smooth and sometimes soft inner surface of the grommet shields the wire from damage.
Grommets are generally used whenever wires pass through punched/drilled sheet metal or plastic casings for this reason. Molded and continuous strip grommets, also known as edge grommets, are manufactured in a wide variety of sizes and lengths expressly for this purpose; they are usually a single piece which can be inserted by hand and used to minimize the transmission of vibration. They were widely used for mounting shock-sensitive computer disk drives, particularly in equipment subject to vibration or jarring. The screws that hold the drive in place pass through grommets that decouple it acoustically from the chassis. Grommets are used in a similar way to acoustically isolate electronic circuit components that are susceptible to microphonism caused by mechanical vibration or jarring.
These are quite common in applications that range from telecom switches and data center cabinets to complex and dense wire/cable and even hydraulic tubing in aircraft, transportation vehicles and medical equipment.