A cable release is an attachment that screws into a shutter release, allowing you to trip the shutter mechanically while being isolated from the camera - to reduce camera shake or to operate the camera from a distance. A cable release is especially useful for long exposures with the camera mounted on a tripod.
Most cable releases consist of a flexible tube, usually made from a spiral of metal covered in rubber, or plastic, sometimes with a fabric outer sleeve. A steel cable runs through the tube, from a press-button at one end to a piston to press the release at the camera end. Some have an additional lock, to hold the shutter open for lengthy time-exposures.
The cable release has largely been replaced by electronic remote releases, which are more suited to electronic cameras, commonly operated wirelessly and communicating using infra-red light.