L.F.O. stands for Low Frequency Oscillator. Such oscillators are used to modulate a property in a synthesizer or effect. It can be used to simulate natural instrument effects, such as vibrato (by applying LFO to pitch) and tremolo (LFO applied to volume), or can be used to change gradually a setting to create special effects, such as applying a LFO to the cutoff value, thus creating phaser-like effect (as this can be done using the LFO in TS404 Channel Settings).