Preston Garvey or Radio Freedom can assign this randomly triggered quest. Head to the indicated settlement and speak to the settler with the waypoint marker above his head. They're having problems with local Raiders, and it's up to you to help them.
Travel to the indicated Raider outpost and kill the targeted enemies. You can also kill the remaining hostiles for additional experience and loot. Return to the settlement with your report, then return to Preston Garvey to give him an update.