Frequency modulation (FM) is what happens when one oscillator controls the pitch of another. The one you hear is the carrier, and the one doing the controlling is the modulator. At slow rates, the pitch rises and falls like vibrato.
Once the modulator speeds up into audio range, your ear stops hearing a wobble and starts hearing new frequencies called sidebands. A plain sine wave turns bright, glassy, or metallic. Simple whole-number ratios between the two oscillators, like 2:1 or 3:1, stay musical and pitched. Ratios that are not whole numbers get you clangorous, bell-like tones.
Modulation depth controls how many sidebands show up. Turn it up for brighter and busier, down for something close to a sine. Try running an envelope to the depth for plucks and bells that mellow as they decay.
