Magnetic plasma transfer was one component of the warp drive aboard Federation starships. It transmitted the plasma to the warp field generators.
When the USS Enterprise-D was immobilized by the Aceton assimilators near Orelious IX in 2366, Geordi La Forge reported that the magnetic plasma transfer to the warp field generators were performing at programmed specs. (TNG: "Booby Trap")
When Leah Brahms came aboard the USS Enterprise-D in 2367, she criticized that the magnetic plasma transfer to the warp field generators was not corresponding to the recommended specs, as Geordi La Forge had adjusted the flow. (TNG: "Galaxy's Child")