Reverberation is the persistence of a sound after it has been produced. It is created when a sound or signal is reflected, causing a large number of reflections to rise up then decay as the sound is absorbed. If you have ever heard your voice linger in an empty gym or any large room, that is natural reverb. Adding reverb to an instrument makes it sound smooth and distant. It is a common effect for vocal tracks as well. This is a great way to create space and depth in music.
Music Production - All You Need To Know 