This massive complex was present on a planet completed dominated by Borg technology and was within a special large chamber. It was noted as being responsible for the coordination of the Borg and responsible for the Collective and its many branches. The Earth probe that became known as V'Ger was accidentally deposited onto the Central Node long ago where it was modified by the early Borg on that world.
In 2371, the USS Monitor, using a Borg Transwarp conduit, traveled to the Borg homeworld where Captain Jean-Luc Picard and Captain James T. Kirk overloaded the Central Node causing its destruction. It was believed that the act of destroying the Central Node would bring about the destruction of the Borg, by essentially severing the network that allowed the individual branches of the Collective to communicate with each other and thus limit the threat they would pose in the future. (TOS novel: The Return)