The meter will indicate the frequency from a power generator. Incoming sine waves are converted to square waves by the 100K resistor and the 6,8 V zener. The square wave is differentiated by the capacitor and the current is averaged by the diodes. The average current is almost exactly proportional to the frequency and can be read directly on a 100 mA meter
To calibrate, hook the circuit up to a 60 Hz poweraline and adjust the 5 K pot to read 60 mA.