Here we solder the feedback resistor R1 on the PCB. In combination with R2 it sets the gain of the amplifier.
G = 1 + R1/R2
You might think that just the ratio is important but also the absolute values are. Resistors generate noise which is the higher the higher their values. For a low noise design you would like to have low resistance values. However, then the OP-amp can only drive a certain load. As a rule of thumb the lowest value for the feedback resistor should be 4.7K. This also keeps the power consumption of the amplifier within reasonable limits because the voltage divider R1/R2 obviously draws power from the OP-amps output. Here I have chosen 100K for the feedback resistor to keep the power consumption low.
Note that the capacitor C2 is not mentioned in the video clip. It acts as a 1st order lowpass filter with a cutoff at about 1kHz. It acts as an anti alias filter for the sigma delta ADC (USB-DUX sigma).