A bushing serves two primary roles. In suspension systems, it is a rubber or polyurethane block encased in a metal sleeve. Pressed into control arms and sway bars, it lets the metal arms pivot without grinding together, absorbing road vibrations to keep the cabin quiet. In engines and starter motors, a bushing is a soft bronze sleeve bearing that supports rotating shafts, protecting harder metals from wear.
Rubber suspension bushings rot over time from road salt and oil leaks. They crack, tear, and harden, allowing suspension joints to shift out of alignment. You will hear a loud clunking sound when driving over bumps, and the steering will feel loose and unstable. Polyurethane replacements are popular because they are stiffer and last longer, though they can squeak if not greased regularly.