It's a threaded "Quick" connector. It's to connect the heater cores supply and return pipes/hoses assemblies. The manufacturers started using thin wall piping with flexie hose permanently attached to it. The thought was to be able to run everything closer to each other with no fear the rubber hoses would burn with the thin wall pipes. What happened was these connectors were made of real inferior pot metal and corroded away quickly. Problem being the connector corrodes off right above the threads. They are a real ***** to get them out of the intake or water pump. The replacement ones are of a higher grade metal and don't corrode away. This is a very difficult repair and not for beginners.

on that 5.4 engine it is a heater hose connector and is kinda unique to your vehicle. the small piece that broke off is the air bleed that returns water to the coolant tank. it can be bought at the dealership or at a used parts place but probably not available at aftermarket parts stores.

Sorry the vehicle is a 99 Lincoln Navigator. You can see in this picture where the piece goes. Its the hole just behind and to the right of the red part on the alternator. I want to say it screws into the upper intake manifold. Is it some sort of vacuum line fitting where the water would spray out of when it broke?