This is an acronym for "Nationella Väg DataBasen" a swedish road database, read more at: https://www.trafikverket.se/tjanster/system-och-verktyg/data/Nationell-vagdatabas/
It is availiable as "Trafikverket Road Network" in both iD and JOSM and can freely be used to draw from.
As of 2018 there is a lot of roads in NVDB missing in OSM. Feel free to help! (and remember specifying source=nvdb or noting in the changeset)