A temporal beacon was a device used to transmit one's location through time. It was used by time travelers so they could be located and retrieved by somebody in another time period.
A temporal beacon was found in 2152 aboard a 31st century vessel by Commander Trip Tucker and Lieutenant Malcolm Reed, and was incorrectly identified as the black box. When it was activated, the beacon, the ship, and the corpse of the pilot vanished, presumably back into the future. (ENT: "Future Tense")
The Xindi-Reptilians who were transported from 2153 to 2004 by the Sphere-Builders were supplied with a temporal beacon so they could be retrieved when they were done with their mission to create a bio-weapon. The beacon was destroyed by Captain Archer, who had been transported back in time by temporal agent Daniels, leaving the Reptilians with no way to return to their own time. Archer and T'Pol later used a beacon which Daniels had supplied them with, to signal the temporal agent that they had completed their mission. (ENT: "Carpenter Street")