A two-quadrant multiplier performs a simple version of amplitude modulation (AM), where that varies the amplitude or loudness of one signal known as the carrier (typically an audio signal, swinging both above and below 0 volts) with a second signal called the modulator. In the typical amplitude modulation (AM) scenario, a low frequency oscillator with a positive voltage (say, between 0v and 5v, or maybe something smaller such as between 1v and 2v) is fed into the control input of a voltage controlled amplifier to add vibrato to an audio signal passing through it. Any negative swings in the modulation signal are ignored; when patching tremolo, you may need to make sure an offset voltage is being added to your LFO to make sure the sound doesn’t cut out on the lower excursions of the LFO’s waveform. (The case where the modulator’s negative as well as positive excursions are used is referred to as a four quadrant multiplier.)
You can do audio rate amplitude or two-quadrant modulation. The result gives you the sum and difference of all the harmonics in the carrier and modulator harmonics, at half the strength of ring or four-quadrant modulation. However, with two-quadrant modulation the carrier is also present, at full strength. Click here for the full technical explanation.