This patch shows the classical FM synthesis technique developed by John Chowning. It's nothing but an oscillator with vibrato controlled by another "modulation" oscillator. First, to understand the patch, set carrier frequency to 400 or so, modulation frequency between 5 and 10, and try modulation index values between 0 and 400, say. You'll hear a sine wave with vibrato.
To get the FM sound, set all three of carrier frequency, modulation frequency, and modulation index in the hundreds. Note that you get a timbral change as you sweep modulation index, because this changes the amplitudes of the components of the output sound but not their frequencies.
The component frequencies are equal to the carrier frequency, plus or minus multiples of the modulator frequency. A more complete discussion of FM occurs in part 5 of this series.