In a synthesizer, A LFO is a low frequency oscillator, which can be set to oscillate (think of the verb) parameters like filter cutoff, resonance, volume, etc. Furthermore, the depth (change between highest and lowest parameter) and frequency (how long it takes to go from highest to lowest). When the frequency is cranked up, you get the kind of wailing fuzzy effect used a lot in trance and other electronica style music.