A test light is an electrical tool used for seeing if a wire has electricity passing thru it. kinda looks like a screwdriver with a light bulb and a wire coming out of it. one end of the wire touches the negative post of the car battery and the 'screw driver' tip touches the wire that you are wanting to know if it is electrified...then the light bulb in the 'screw driver' will light up if it is good.