The Ocampa city was the subterranean settlement, kilometres beneath the desert surface of Ocampa, where Kes's people lived in sheltered dependence on energy sent by the Caretaker.
After Nacene explorers accidentally stripped the planet of its ability to produce rain, an event the Ocampa remember as the Warming, the Caretaker moved the survivors underground. The city he built for them had food dispensers, water from deep reserves and power delivered from his array.
In Caretaker, Harry Kim and B'Elanna Torres are sent there to be nursed after the entity's experiments leave them ill. Most residents accept their comfortable confinement, but Kes and a few others question it and explore the old tunnels to the surface.
As the Caretaker nears death he sends a final surplus of energy and seals the conduits to protect the city from the Kazon. Kes leaves with Voyager.