This Socket Tester works for most European grounded sockets, not for the UK 3 hole sockets (for this you'll need a grounded country-specific adapter).
With this socket tester you can easily test if your grounded socket is working, so if it is correctly connected to the ground via the ground loop or ground pin of your home.
You can easily identify a grounded socket by the metal strips on the side (Dutch, European) or the metal pin that sticks out (Belgian, ...). If you do not see metal, your outlet is not grounded. Then it makes no sense to use this tester.
Download here the Test Chart for the Socket Tester