A "monad" is a metahuman whose mind and personality have been overwritten by an artificial intelligence via a nanite infection. The term "cognitive fragmentation disorder" (CFD) was coined before anyone really knew what was happening, but that's the result.
"NV" is a special attribute called nanite volume, a measure of the monad's strength.
See:
SR5 Stolen Souls ("The CFD Virus," p.192-195)
SR5 Lockdown ("Cognitive Fragmentation Disorder Virus," p.198-203)
SR5 Dark Terrors ("Monads and CFD," p.70-87)
SR6 Collapsing Now ("The Monads," p.64-78)