The Stetzer meter simply plugs into any standard UK socket and measures GS (Graham-Stetzer) units of dirty electricity in your mains electricity supply, displayed as a number in the range of 0-1999.
It does this by measuring the rate of change of voltage, in the frequency range 2kHz to 150kHz.
Less than 25 GS units = good
25 to 50 GS units = average
More than 50 = undesirable
It is designed for use with the Stetzer filters which are designed to reduce the GS readings
The meter is especially useful when fitting Stetzer filters, to determine the best places to plug in the filters to get the maximum reduction in dirty electricity on the mains.