An injector assembly is a crucial component of a starship's main propulsion systems. As the name suggests, the injector assembly is presumably comprised of an array of smaller components. The injector supplies a substance (possibly deuterium) into a vessel's warp core or impulse drive reactor as a function of the matter/antimatter annihilation which generates a ships warp field and other power systems.
When the crew of the Enterprise NX-01 needed to arrive to meet Degra at a prearranged time and place but were too far away to get there on time, Lorian modified the Enterprise's injector assembly to reach up to warp factor 7 but at shorter intervals. (ENT: "E²")