A circuit that arithmetically adds two voltages, producing an output signal whose voltage is the sum of the two input voltages. For instance, to take a two-input adder, if one input is at 1.7V and the other is at 0.5V, the output will be 2.2V. An example usage for a voltage adder is to (assuming a volts/octave synth architecture) process a control voltage output from a sequencer that is driving a voltage controlled oscillator to play a repeating melody. By adding, say, a control voltage from a keyboard to the sequencer's output, the performer can transpose the sequence to different keys by pressing a key on the keyboard.
The voltage adder is a simple circuit which has a number of uses in a volts/octave synth architecture. A voltage subtractor is also not difficult and equally useful. Conversely, a volts/Hz architecture requires a far more complex voltage multiplier circuit for the same purposes, which is one main reason why volts/octave architecture is preferred in most analog synths.