An LFO or Low-Frequency-Oscillator, is a special type of oscillator that operates below the threshold of human hearing (usually below around 20 Hz). It is used to alter parameters in instruments or sounds to create audio effects such as vibrato or phasing, or to add perceived movement to a track.
Most modern hardware and software synths have an LFO built in. Within the instrument they can then be used to change or modulate some aspect of the sound over time. For example, they could be assigned to cut-off frequencies, pitch, volume levels, or even the waveforms themselves.
See below for a simple example inside the VST plugin Diva from U-he. Here the cut-off frequency is being modulated by LFO 2 to give the sound it's phasing effect. Obviously changing parameters of the LFO such as the rate and depth will drastically alter the end result.
Some DAWs also have their own built-in LFOs. In Ableton's case it has its own Max for Live LFO device. This comes in really useful when wanting to modulate parameters on something that doesn’t already have its own inbuilt LFO.
In the simple example below you can see that Ableton’s LFO device has been mapped to the Decay of the Hi-Hat. The subtle LFO changes the sound of the Hi-hat over time, and combined with some saturation and a touch of flanger, gives more movement and adds interest to the track:
There are many other different creative ways in which an LFO can be used. Assign one for example to the panning of a sound or instrument to have the stereo field change over time. Control a low-pass filter to make a dubstep style wobble-bass, or create an evolving ambient sound by applying a slow rate LFO to a synth or guitar track.
I hope you enjoyed this short introduction to LFOs. Have a great day and happy producing!