An injector assembly was, as far as Terran technology was concerned, a crucial component of a starship's main propulsion systems. As the name suggests, the injector assembly was 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")
The Son'a injector assembly
Two centuries later, in 2373, an injector assembly was for the Son'a a crucial piece of equipment in order to harvest metaphasic radiation from the rings of the planet Ba'ku. That piece of equipment was to be launched from a specialty ship, the Son'a collector, right into the rings of Ba'ku where it was to be detonated by firing charges that would start a thermolytic reaction, the radiation of which to be collected by he collector. A simulation ran by Ahdar Ru'afo showed that it was a viable option. Nevertheless, launch and execution were prevented through the intervention of the USS Enterprise-E and its crew. In the process both collector and assembly were destroyed. (Star Trek: Insurrection)