A broad-line region (BLR or broad emission-line region) is an observed region around an active galactic nucleus showing considerable broad emission lines. The general view is that it is a region near the SMBH's accretion disk with numerous gas clouds (BLR clouds or broad-line region clouds), and the broadening is Doppler broadening of spectral lines from these clouds, due to the clouds' high velocity, a must for any clouds remaining in the high gravitational field of the SMBH.
Reverberation mapping is a method of estimating the mass of the SMBH using the velocities suggested by the doppler broadening and the size of the region. The size is determined by the time it takes for AGN luminosity changes to induce corresponding changes in the broad lines (the induced changes being referred to as reverberations). The delay is attributed to the speed of light and taken as an estimate of the radius of the region, which with the velocity distribution, allows the mass if the internal object can be estimated. The method makes use of the fact that AGNs vary in luminosity.
An AGN may also possess a narrow-line region (NLR) interpreted as material further from the central SMBH, and it may have both.