REVERB stands for reverberation. This effect lets you tailor the spacialization of the sound.
REVERB: toggle the checkbox to engage/disengage Reverb effect.
Room size: use this slider to adjust the room size for reverb.
Dry: use this slider to control the level of unprocessed or dry signal.
Wet: use this slider to control the level of reverb or processed signal.
Width: use this slider to adjust the stereo image of processed signal.