Both opamps have a gain of -2 V/V. Opamp U1 has a ground-referenced input and output, meaning that it needs positive and negative supply voltages to work properly. But, opamp U2 provides a 2.5 V reference, meaning that the input is compared to 2.5 V, amplified by -2 V/V, then superimposed on top of 2.5 V at the output. All of this can work within a 0 to 5 V range, which means a negative rail is no longer needed.
The input is 1 Vpk @ 1 kHz.
