A low frequency oscillator is an oscillator with a frequency below the human range of hearing, below 30 Hz or so. We can't directly hear a low frequency oscillator. A low frequency oscillator can be any wave shape. Most common is the sine wave. We can use square, triangle, rectangle, sawtooth, and just about any other form as a low frequency oscillator. Like any other oscillator, low frequency oscillators have the parameters of shape, amplitude, and frequency.
The purpose of a low frequency oscillator is to change a parameter, or many parameters, of either a sound generator or sound modifier. For example, we can put a low frequency oscillator as the source of a cutoff frequency to a filter. We will discuss common use cases of low frequency oscillators in the "modulation effects" unit. By changing parameters over time, we can add interest to our music. Change is good!