The Prefab Space Station is the second "node" in the Lunar Transfer System, the first being the Earth surface base (Cape Canaveral). It serves as a waypoint between the surface-LEO leg and LEO-Moon leg of the journey. Payload modules traveling from Earth to the Moon or back can be stored here temporarily while they await pick-up by a Lunar Transfer Vessel or a Space Shuttle orbiter. It is also the home away from home for the crews of the LTV, and a place for translunar passengers to get a stretch while they await their connection.
Some scenarios require the following addons, although you can always insert the Prefab Station into your own scenario files:
Fuel Managment MFD, found on Avsim:
The Lunar Transfer Vessel and various payload modules, which you can find on Orbithangar by searching my username (Andy44):
The Nova-SSTO last seen on Sourceforge: