LFO is the abbreviation for Low Frequency Oscillator. An oscillator is primarily used as a modulator for other things. Low frequency oscillators may or may not operate exclusively below 20 Hz (the lower limit of typical human hearing), but by definition they are not designed to be used as sound generating elements, even though they can be in some synthesizers. When you bring in modulation (vibrato) in a keyboard, or a chours in an effects processor you are using an LFO to generate the waveform that produces the variance. In synthesizers and more advanced effects units LFOs can often be routed to many different parameters (often simultaneously). They can sometimes generate different waveforms (sine, sawtooth, square, random, etc.) for different types of modulation effects.