The ZPM Hub is a device present in Ancient city-ships that serves as a means to draw power from Zero Point Modules and ultimately supply it throughout the city through a series of power conduits.
The device is roughly triangular in appearance. The top of the device contains three slots into which one ZPM each may be inserted. When initially placed into the hub, a ZPM will illuminate, indicating that a connection has been made between it and the hub. However, power will not begin to flow until a command is given from one of the hub's adjacent computer terminals. Once such a command has been given, the ZPM will lower into the device, with the top of the ZPM becoming flush with the top of the device. (SGA: "Before I Sleep", "The Siege, Part 3")
Behind the scenes
The prop for the hub was re-used in "Progeny" as some kind of computing terminal that the Ancients were using to create the Asurans. The Zero Point Module slots were tinted blue to differentiate them.