Reverb is used to simulate spaces such as rooms, churches, chambers and halls etc, this adds depth and width to a mix.
When we record any sound the sound that is being recorded is in mono, mono mean one, this doesn't have any stereo information on it so we can create this by adding reverb to it.
The earliest reverb that was around was plate and spring reverb, the 80's then introduced digital reverb.
There is another way to simulate reverb this can be done by placing a speaker in a room that has good acoustics, this with setting up two microphones 180 degrees out of phase with each other and then by sending signal to the speaker and recording what the microphones pick up this created a true room reverb.
Overall I could write a whole book on reverb the basic controls are below.