Monofilament is the common contraction for "monomolecular filament", a long, strong chain of molecules that effectively create the sharpest possible physical cutting edge, allowing them to cut through virtually anything. No form of defense was known to repel monofilament strands.
The Federation is aware of the technology; however, they have been incapable of utilizing it as the strands become unstable and collapse.
The Hive species had developed the technology to replicate and maintain monofilaments. They were able to use such technology as a form of weapons in their fighter craft by maintaining a monofilament strand between them and heading towards enemy starships, which would be ripped apart regardless if shields were functional or not. They developed this even further with the development of monofilament torpedoes which would explode and shower a region with the strands, capable of destroying a large number of enemy starships that were clustered together. (DS9 novel: Objective: Bajor)
The Romulans bound the wrists of prisoners tried before the Senate and Praetorate with silk ribbons containing monofilaments, which would cut off their hands if pulled on. (TOS - Rihannsu novel: The Romulan Way)