Nacelles, also called Warp Nacelles, are one of the vital components of Faster Than Light travel.
Nacelles use plasma created by the Warp Core to create a warp field, which is also sometimes called a warp bubble or shell. The size of the nacelle determines not only the size of the field but also the warp velocity that can be sustained. No vessel may have nacelles longer than it's own length.
Most often they are used in pairs, though some Starship classes have been designed with odd-numbers of nacelles. Note that adding nacelles to a vessel does NOT increase its warp rating. It would only increase the size of bubble that could be created, thus allowing it to enclose another ship if necessary.
The primary component of the Nacelle is the Warp Coil which uses the plasma stream into a warp field. The size of the Warp Coil determines not only the size of the warp bubble, but also how fast the vessel is able to travel at FTL speeds - it's Warp Rating.