Fuel Rail Insulator
Fuel Rail Insulators are small rubber rings installed between the fuel rail and the lower intake manifold. These help create a strong seal between these two parts in order to prevent a vacuum/boost leak. Like Balance Tube O-Rings, they are generally reusable, but should be replaced when they've become too "squished" to create a good seal.
Please note that these should not be confused with Fuel Rail Grommets, which support the fuel rail at its mounting points.