A rear derailleur is a crucial component of a bicycle's gear system. It is responsible for shifting the chain between different gears on the rear cassette, allowing the rider to change resistance and speed. The derailleur moves the chain laterally to align it with the desired gear, ensuring smooth transitions while pedaling. Typically mounted on the bike's rear triangle, the rear derailleur consists of a cage that holds the chain and a mechanism that moves the cage. It is controlled by the rider through shifters located on the handlebars, making it an essential part of modern bicycles for efficient riding.