It’s fun to twist the knobs on the front panel of a synthesizer back and forth to directly change its sound. However, sometimes you wish you had a device that would do the twisting for you, so you can do other things like play notes or adjust other controls. The most common type of module that does this is called a “modulator”, and the most common modulator is a “
Modulation sources including LFOs are explained on page 20 of Patch & Tweak; LFO modules and their distinguishing features are presented on pages 232-5. Get the book here.