Basically that it's an old and largely discredited notion that the centuries directly following the fall of Rome Europe was plunged into a period of regression

Basically that it's an old and largely discredited notion that the centuries directly following the fall of Rome Europe was plunged into a period of regression and backsliding. It's an ambiguous term in that it can mean that the times themselves were "dark" as in grim and bad, or that the records from that time are few, and therefore visibility in the historical record is "dark" or lacking.