The Reverb is used to create a sense of space surrounding the audio, to give it a room in which it lives. Reverb is short for reverberation and is the sonic signature of a space as sound bounces around in it. Reverb is heard all over in everyday life, like the sound of your footsteps in a tunnel or the roar of a bear in a cave.
Set the size of the room. The higher the value the longer is the tail of the reverb.
Use this to control the dampness of the reverb tail. Lowering the the value of this knob will give the reverb a darker sound. Much like the LPF filter on the Equalizer.
Use mix to control the balance between the original sound and the signal of the reverb.