A reverb is a unit which applies reverberation to a signal. In the days before sophisticated DSPs (digital signal processors), there were two ways of gererating reverb.

The first was to put the signal into a reverberant space and place a mic to record the reverberation. So you could either sing in your bathroom, or put a speaker replaying your voice in the bathroom and record the result.

The second was to suspend a large metal plate on springs, and feed a signal into one end and take it out the other. These plates gave a characteristic sound, hence the 'Plate' reverb programs often found on todays units.

Reverb units are now small, affordable and highly sophisticated. The more expensive ones give precise control over a huge number of parameters, so much so that most users seldom get beyond the factory presets which give a huge number of differing reverb types, from small rooms to grand canyons.

If changes are to be made to a program, it is usually the decay time (the time it takes for the reverb to fall below a certain level) and the amount of high frequency in the reverb sound that make the most difference.