The Slipbeacon is a method of supposedly faster-than-light interstellar communication via Slipstream Space used by the UNSC. Compact enough to be mounted on a starship, the Slipbeacon is known to be used in distress situations. Beside messages carried by starships in shipboard memory or Slipspace COM launchers, it is the one of the few methods of faster-than-light communication known to the UNSC, though its exact nature or function is unclear.
When the rear half of the UNSC Forward Unto Dawn was left stranded in space after the Battle of Installation 00 with John-117 and Cortana on board, Cortana mentioned that she would "drop a beacon" in the hopes of attracting a rescue party. Given the circumstances, this may well be referring to a Slipbeacon.