FM synthesisFM stands for "Frequency Modulation". Altering the frequency of an oscillator very rapidly within its duty cycle will create new waveforms with increased harmonics.
The simplest way to build an FM setup is to connect 1 oscillator to the pitch input of the next oscillator.
Due to drifting effects, FM is generally not really feasible on an analog modular synth. (can still give interesting noises though!)