An interstellar relay was a type of communication relay station operated by Starfleet Communications in the mid-23rd century.
During the 2250s, Starfleet had an exemplary reputation for tech hygiene, as evidenced by it consistently sending a ship to fix any relay that went down. In 2256, the USS Shenzhou was sent to investigate damage to an interstellar relay located in a binary star system at the edge of Federation space. It was eventually found that the Klingons had damaged the relay to lure in Starfleet. (DIS: "The Vulcan Hello")
There were interstellar relays orbiting Earth. (DIS: "Will You Take My Hand?")
StarTrek.com identifies this technology as an "interstellar positioning relay" and states its purpose to be monitoring the remote boundaries of Federation space.