The circuit uses a small resistor network of just a feedback resistor and resistors for the input signals. The output is the summation of the contribution from signal 1 (orange) and signal 2 (green).
The blue output voltage is represented as Vo = - V1 * ( Rf / R1 ) - V2 * ( Rf / R2 ).
Changing the value of the feedback resistor Rf can change the total output, while varying the input resistors can affect each inputs total contribution to the output.
