“It is a pale shadow of a once pivotal and very active machine.”
This ancient structure is an Accelerator. Specifically it is a Fusion Coil Accelerator (a.k.a. Coilgun or Railgun). It is no longer functional. Now it serves as a useful landmark Guardians can use to navigate the Moon.
During the Golden Age this Accelerator was used to deliver cargo to aid the colonization effort. It could launch cargo at extremely high speeds throughout the Solar System. It would arrive at its destination in a matter of minutes or hours depending on the location. Specific catch pads were constructed to handle the arriving goods.