A Subspace Relay is an object which receives subspace communications signals and retransmits them. Signals sent in the subspace realm travel faster-than-light (FTL). Over vast distances, such signals will eventually emerge into normal space, dropping to very slow light speed, which would make interstellar communications very nearly impossible. Subspace relays solve this problem by receiving FTL subspace communications before they emerge into normal space and retransmitting them in subspace at higher energies, allowing the signal to travel further in subspace, and to arrive at its final destination much faster, than it could without the relay.
See also subspace amplifier.