This circuit is also known as a 60 Hz band reject filter with gain. This uses a twin-T notch filter to attenuate any noise at 60 Hz. This 60 Hz noise is due to the mains hum - that is noise generated from high current carrying conductors like the mains. The 60 Hz noise signals are attenuated completely but quality is mediocre. Varying the R6 and R7 resistor ratio will vary the quality factor of the circuit.