This achievement is for paying tribute to each of the 5 raider factions at least once during a "session" (a session is just the current game you're playing).
The five raiders are:
Lynn the Queen
The Unshakable Ago
The Nihilists
Liam The Prophet
Mike the Mega-Devourer
You will know if raiders are coming due to a pop-up from a settler, and a shield icon at the top centre of the screen, as well as a red ! on the map. Once the red ! appears you can save, and if you get a raider you already bribed, reload. It should be a different one.