This is a pulley driven unit that balances out harmonic vibrations from the crankshaft. Typical symptoms of a bad dampener are unusual noises from the passenger side of the engine bay.
The dampener's rubber layer can dry out and deteriorate after years of exposure to oil, dirt, and temperature extremes. During replacement, it’s wise to replace the hex bolt as well. The serpentine belt must be removed to access this part.
Also known as an Engine Harmonic Balancer or Vibration Dampener.