A PTFE liner is a bonded layer of PTFE (Teflon) — typically a fabric or resin-matrix composite — installed between the ball and housing of a rod end or spherical bearing. As the ball slides against the liner, microscopic amounts of PTFE transfer onto the ball’s surface, creating a low-friction, self-lubricating interface. A PTFE-lined joint should never be greased: doing so interrupts the transfer film and turns trapped contaminants into an abrasive paste.
See the full guide: Rod End & Heim Joint Lubrication: The Complete Guide