What is a summing amplifier and how does it work?

A summing amplifier uses an op-amp in inverting configuration with multiple input resistors connected to the inverting terminal. The output is the inverted weighted sum of all inputs: Vout = -Rf(V1/R1 + V2/R2 + V3/R3...). When all input resistors are equal, it produces an inverted sum of inputs. It is used in audio mixers, digital-to-analog converters, and signal combining applications.