The Jedi beacon transceiver was an encoded subspace receiver beacon carried by members of the Jedi Order, allowing them to receive signals and broadcasts from the Jedi Temple on Coruscant.
Beacon transceivers were small, portable devices with a display screen and communications transceiver. The transceiver picked up broadcasts from the Jedi beacon on Coruscant or from any Jedi's personal comlink. In emergencies, a Jedi would broadcast Emergency Code Nine Thirteen to any beacon transceiver in range, calling for immediate assistance. These devices also picked up recall and retreat signals from the Temple, commonly when HoloNet channels were compromised.
When Coruscant was attacked by General Grievous's forces, Yoda transmitted a recall message to all Jedi transceivers.
During the execution of Order 66, Darth Vader, having conquered the Jedi Temple, set the Jedi beacon to call all Jedi home - into the ambush awaiting at the Temple. Any surviving Jedi received the priority signal on their transceivers. However, Jedi Masters Obi-Wan Kenobi and Yoda managed to infiltrate the Temple after Vader left for the Mustafar system. Kenobi recalibrated the beacon, warning all Jedi to stay away from Imperial Center.