Synth sounds would be too static (and thus boring) if there were no modulators like envelopes. To get even more spice into a sound, synth programmers use low frequency oscillators, in short LFOs.
The Minimoog provides three ocillators. The third one often was used to apply a vibrato to the sound. A vibrato effectively means to modulate the pitch of the main oscillator(s), thus called frequency modulation aka FM.
Most modern synths provide two, three or even more LFOs which can be routed to almost arbitrary destinations, not only to the main oscillators’ pitches. Use cases often found are filter cutoff modulation, volume modulation (aka tremolo), panorama modulation and the like.
LFOs basically offer similar waveforms as the main oscillators. The most important ones are sine and triangle waves. Most often also sawtooth and squarewaves are supported. Special waveforms include random and sample & hold waveforms, the latter one more or less being a random waveform as well.
Some synths provide the aforementioned basic waveform as additional exponential waves, granting less synthetic sound changes. For the same reason the LFOs’ waveforms can be changed concerning their contour (Access Virus) or to modulate the symmetry (Waldorf Microwave I), to give the sound more living and breathing.
LFOs often provide a fade in time. Some machines allow the LFO 1 to sync each other to put them into the same phase. Additionally modern synths offer sync capabilities to the MIDI clock, so the LFOs can perform modulations according to the speed and rhythm of a (software) MIDI sequencer.
The speed of LFOs often is less than the audible range. However, modern software synths provide speeds which clearly reach the audible area. This allows the creation of interesting sounds using frequency modulations of the main oscillators or the filter cutoffs.
Using LFOs is a complex topic and a source of endless hours of joy and pleasure. It heavily depends on the type of sound to create how to use them. Thus this posting remains what it is: a bird’s eye view on LFOs.