Ring modulation is an audio effect performed by multiplying two audio signals, where one is typically a sine-wave or another simple waveform.
Multiplication in the time domain is the same as convolution in the frequency domain. Thus, in the basic case where two sine waves of frequencies f1 and f2 (f2>f1) are multiplied, two new sine waves are created, with one at f1+f2 and the other at f2-f1.
On the C64 SID chip, ring modulation multiplies a triangle wave with a square wave. A ring modulator was a common feature on early modular Moog synthesizers. The ring modulator went out of fashion with the advent of all-in-one synthesizers sampled-based synthesizers, but has returned as a feature in digital modelling and software synthesizers.