Silver is not magnetic. However it is 'diamagnetic' - a sliding magnet sets up an 'electrical field' which you can detect with suitable magnets.
These magnets are 'suitable' because of their size and power, read all about it.
Copper reacts the same way as silver (though the effect isn't as strong), so this test will not tell you if you have a copper coin plated with silver. YOU STILL NEED A SILVER TESTER to be certain.