Reverberation mapping is a technique used in astronomy to study the inner regions of active galactic nuclei (AGN) and quasars. It involves measuring the time delay between variations in the brightness of the central source of the AGN and the emission from the surrounding gas clouds. This time delay is caused by the light from the central source traveling at different speeds and is used to map out the structure and dynamics of the gas clouds near the supermassive black hole at the center of the AGN. Reverberation mapping can provide valuable insights into the size, shape, and movement of the gas clouds, as well as the mass and spin of the black hole itself.