It is a call for help received by a runaway slave named Wernher who has just escaped from the treacherous industrial slaving town The Pitt. Tracking down the origins of this broadcast leads you to Wernher at the north end of the map in a fight with 3 Pitt raiders. This also triggers the quest Into The Pitt.
If you are inside Operation: Anchorage's Anchorage Reclamation simulation, this radio signal is the only one available. The signal can also be heard perfectly clearly from Point Lookout and Mothership Zeta (due either to the size of the radio tower used, or to the same bug/oversight that allows it to be heard in the Anchorage simulation).