Warp nacelles are the long engine pods on a starship that contain the warp coils, which shape the energy from the warp core into a faster-than-light warp field.
Plasma from the warp core flows into each nacelle, where rows of coils convert it into the layered field that carries the ship. Placement of the nacelles matters, since they must have a clear line of sight to each other to form a stable field, which shaped the distinctive look of Starfleet designs.
By the 2370s some ships, such as Voyager, used variable-geometry nacelles that pivoted into a raised position before the ship went to warp.