Adding different voltages together, while also being able to "weight" each input differently. This is also great when combining voltages of different frequencies.
A summing amplifier has two or more inputs, and its output voltage is proportional to the negative of the algebraic sum of its input voltages.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
A summing amplifier is an op amp circuit that combines several inputs and produces an output that is the weighted sum of the inputs.
Fundamentals of Electric Circuits, 5th Edition by Charles K. Alexander and Matthew N. O. Sadiku
A summing amplifier sums several (weighted) voltages: