What is a Test pen or Voltage tester?
This tool is used to test circuits quickly to ensure that the circuit or conductor is live or not guided from indicator bulb or LED.The rated voltage for this tool only for 600VAC below.DONT USE IT FOR TEST HIGH VOLTAGE.It can cause the serious injury.
If a bulb is light on,that mean the conductor or a circuit is live.Usually it is shaped like a Screwdriver slotted, but it has the indicator in it.It can be used as the Screwdriver and the tools to test live circuits.