From wiki.flightgear.org
The
tail rotor of a helicopter is mounted on the tail of a traditional single-rotor helicopter, close to perpendicular to the main rotor. It is primarily used in order to counteract the yaw motion and the torque that a rapidly turning disk naturally produces. The tail rotor in simple terms is a
propeller that pushes the body of the helicopter in the direction of the main rotor, preventing loss of control.