In Nightfall, Lagash is a world of never-ending sunlight. However, every two-thousand-and-some-odd years, darkness falls. Or, at least, that's the prediction made by the religious fanatics of the world. Scientists who studied heliocentrism were laughed at, much like here, for a while. Those who proposed anything beyond that, or corroboration of the fanatics' theories, were debunked—until they prove the theories right.
When darkness falls, it destroys the long-held beliefs of Saro City citizens. They are not able to deal with the consequences of the scientific revelation that there are innumerable other stars and planets in their universe or that darkness can be withstood. Because of the fall of night and its destruction of those core beliefs, the foundation of their culture is violently shaken. People are driven mad, and with that, the civilization is in danger of falling into disarray.
There are plenty of parallels and allegories to be made. The last sentence is open to interpretation. Finding your own is half the fun.