Relay stations were facilities built by the Forerunners on their Halo fortress worlds. An example is Installation 05, with such facilities being part of a network that has kept the ring functioning for many centuries.
The only such facility encountered so far on Delta Halo consists of one large spinning center section which appears to be a some sort of dish receiver, a tall structure with many floors and an antenna-like structure on top, although the top can not be reached. Around the main structures, there are many other smaller structures and walkways that lead off from the middle, most of which lead back to the center. These stations seem to be the Forerunner equivalent of the human communication relay.