What is reverberation and how can …

The waves suffer multiple reflections from the walls, ceilings and other materials present, when a sound is produced in a big hall or auditorium. As a result of this, sound persists for some time even after the source has stopped producing sound.
Revertebration is the process of persistence of sound due to repeated reflection. 

Methods of reducing reverberation.

In a big hall or auditorium, excessive reverberation is highly undesirable. The following below methods can be used to reduce them.

(i) By covering the walls and roof of the auditorium with sound absorbent materials like compressed fireboard, rough plaster, etc.

(ii) Providing open windows in the space.

(iii) Providing heavy curtains with folds, so that the sound gets absorbed. 

(iv) By using good sound absorbing materials for the seats.