It is a call for help received by a runaway slave named Wernher who has just escaped from the Pitt. Tracking down the origins of this broadcast leads to Wernher at the north end of the map in a fight with three Pitt raiders. This also triggers the quest Into The Pitt.
"To anyone who can hear me: my name is Wernher. I come from a settlement to the north. I have information of great value to anyone willing to help me free my people. Please... help us. This message repeats."
The signal can be heard in all add-on locations.