A rear derailleur is a crucial component of a bicycle's gear system. It is located at the back of the bike and is responsible for shifting the chain between different gears on the rear cassette. This allows the rider to change gears smoothly, making it easier to pedal on various terrains. The rear derailleur works by moving the chain laterally across the gears when the rider shifts the gear lever. It is controlled by a cable connected to the shifter, which pulls or releases the cable to adjust the position of the derailleur. Proper adjustment and maintenance of the rear derailleur are essential for optimal bike performance.