A primary warp coil was an integral component of warp-powered starships.
In 2154, Enterprise NX-01's primary warp coil was damaged beyond repair due to a Xindi attack at Azati Prime. In order to make a rendezvous with Degra on time, Captain Archer was forced to mount an attack on an innocent Illyrian starship for the purpose of stealing their warp coil. Without the coil, the ship was estimated to take at least three years to return to their star system. (ENT: "Damage")