Part of a rack-and-pinion steering system, the rack is a bar parallel to the front axle that moves left or right when the steering wheel is turned, aiming the front wheels in the correct direction. The pinion is a small gear at the end of the vehicle’s steering column that engages the rack. Though steering racks are hearty, they sometimes require replacement. Signs of a failing rack include difficulty turning the steering wheel in one direction but not the other, jerkiness of motion or noises.
Image courtesy of ClearMechanic.com，essay post from cars.com