LFO
Low Frequency Oscillator — an oscillator too slow to hear on its own, used instead to turn other knobs for you.
An LFO typically runs below 20 Hz, under the threshold of hearing, in sine, triangle, square or random shapes. Aim it at pitch and you get vibrato, at volume and you get tremolo, at filter cutoff and you get the rhythmic wobble that dubstep turned into an entire genre around 2010. Sync it to tempo via MIDI clock and the modulation follows the song instead of drifting against it.
The same idea powers the modulation effects: a phaser, chorus or flanger is basically a filter or delay that an LFO slowly rocks back and forth. Where an ADSR shapes a note once per keystroke, the LFO keeps cycling for as long as the note lasts.