Hyperspace relays were devices (individually known as buoys) used to communicate and send messages through hyperspace. At some time prior to Operation: Influx during the Old Republic era the Galactic Republic decided to shut off all hyperspace relays in communication with all parts of the galaxy except for the Core Worlds, the Colonies and some of the Inner Rim meaning communication with and between all other areas of the galaxy ceased. This was done to prevent the large number of Sith factions outside of this area using the relays for their own conquests of power. It also prevented the Sith from accessing the Republics database of hyperspace lanes. However a few Sith lord managed to create their own rudimentary hyperspace relays sometimes by reverse engineering the shut down Republic buoys.
Hyperspace relays most likely succeeded the hyperspace beacons.