This is a process wherein the sample is changed to make it seem like the music is being played in a large hall. There are instances when it will make the sound seem like it is still there even if the sound produced is already over. Take note that this is something that can be used both in real life and even in music productions.
This makes all the difference when you are trying to produce a sound that seems to have been made professionally with something that is obviously done in one of your spare rooms in the house. The reverb is sometimes used on vocals in order to enhance the vocals of the singer more.