In pedal circuits, the most common functions of a diode are
1) Reverse polarity protection, that is, a device to prevent you from frying your pedal when your second cousin visits and plugs your pedal into an AC/AC adapter, and
2) Clipping diodes for overdrive and distortion circuits. By connecting diodes in opposite directions from the signal to ground, whatever signal passes through that portion of the circuit will get clipped at the forward voltage level of the diodes, with any voltage beyond that going to ground, so your sine wave converts to a square wave.
We have some models of diodes for each function.