A reverberator creates a sense of space. Classic guitar amps use a "spring reverb", which is one or more metal springs through which the sound propagates and disperses. GeoShred uses a more modern design from the field of Computer Music based on delay lines and Schroeder allpass filters
The Wiz Verb is always stereo. It is normally placed last in the Effects Chain.
Increasing the damping decreases the reverberation time.
Level is the "dry/wet mix": 0 for "dry" (no reverb), and 1 for "wet" (pure reverberator output). Note that stereo imaging is normally reduced by reverberation, because the reverberated sound is supposed sound like it's coming from all directions. Reduce the Reverb Level to better preserve stereo imaging.
Increasing the Room Size parameter lengthens the delay lines used in the reverberator, corresponding to longer path lengths between reflections in an acoustic space.