A lunar beacon was a radio transmitter on or around Earth's satellite Luna. The beacon would transmit a signal which aided in fixing the position of a spacecraft.
In the mid-2270s, lunar beacons indicated that V'ger was on a course towards Earth orbit. Starfleet sent a signal, with intelligence obtained from the beacons, to the USS Enterprise. (Star Trek: The Motion Picture)
The Star Trek Encyclopedia (4th ed., vol. 1, p. 490) located these beacons on Luna and described these technological devices as "navigational reference transmitters and transponders...used to help spacecraft determine their position in cisular space." Cisular was probably a misspelling for cislunar.