A reverb effect is essentially a long, complex series of sound delays. When you run an audio signal through a reverb effect, the audio fades away at the end rather than stopping abruptly. This mimics the manner in which sound echoes in a listening space, thereby creating more realistic sound. Different types of reverb and different reverb configurations create different effects: a hall reverb, for example, emulates a large concert space, while a room reverb simulates a smaller, more intimate listening environment.
Monophonic audio contains identical audio signals on the left- and right-hand channels. The two channels in stereo audio, on the other hand, are slightly different. Stereo audio recordings are designed to reproduce the experience of listening to a sound in real life with greater fidelity than mono audio recordings. When you listen to a live violin player, for example, your left and right ears hear the violin slightly differently, as the sound reaches each ear from a slightly different angle and distance. Therefore, stereo usually sounds more realistic than mono.
When you send an audio signal to a stereo reverb effect, the effect splits the signal into left and right channels. It then applies a slightly different reverberation effect to each of the two channels. Stereo reverb effects allow you to configure the parameters of the two channels independently. For example, you can make the left-hand channel of a reverberated audio signal fade away slightly faster than the right-hand channel by decreasing its "Time" setting.
If you produce music using a digital audio workstation on your computer, the program may only include a mono reverb effect. If you want to experiment with stereo reverb without installing a stereo reverb plug-in on your system, you can recreate the effect using two mono reverbs. Duplicate an audio channel, then pan one channel all the way to the left and the other all the way to the right. Add a reverb effect to each channel, then set the parameters of the effects so that they're slightly different. Record the two channels simultaneously to a new stereo audio channel to make your own stereo reverb effect.