The recent observation of nanohertz gravitational waves by Pulsar Timing Arrays (PTAs) has opened a new observational window into the universe. Although the origin of the signal has yet to be determined, the measured gravitational-wave background may be produced by a collection of in-spiralling supermassive black holes (SMBHs) or by physics beyond the standard model. The evidence for this background therefore offers unique insights into the astrophysics of SMBHs and potentially new physics. This Rapid Response Workshop will solely focus on the scientific implications for SMBHs of this first PTA detection bringing together experts on both theory and observations.