The summing amplifier does what its name implies - it produces an output that is proportional to the sum of its inputs.
The output voltage is found by:
Notice that this is an inverting amplifier configuration and the output has the opposite polarity or the opposite phase of the scaled summation of the inputs.
Rb is not required for the circuit to function (it can be replaced with a connection to ground). It is included to minimize the effect of bias currents from the op-amp. If it is included its value should be the parallel combination of the input resistors (R1 through Rn) and Rf.