The test light is simply an electric lamp connected with one or two insulated wire leads. Often, it takes the form of a screwdriver with the lamp connected between the tip of the screwdriver and a single lead that projects out the back of the screwdriver. By connecting the flying lead to an earth (ground) reference and touching the screwdriver tip to various points in the circuit, the presence or absence of voltage at each point can be determined and simple faults detected and traced to their root cause.
For line voltage (mains) work, the lamp is usually a small neon lamp connected in series with an appropriate ballast resistor. These lamps often can operate across a wide range of voltages from 90V up to several hundred volts. In some cases, several separate lamps are used with resistive voltage dividers arranged to allow additional lamps to strike as the applied voltage rises higher; with the lamps mounted in order from lowest voltage to highest, this minimal bar graph provides a crude indication of voltage.