From the back of the box:
A range of friends and foes, with game characteristics and backstories, providing additional NPCs and replacement investigators (48 cards).
Call of Cthulhu Keeper Decks are designed to provide in game [sic] resources, handy references, and information. Use these decks to supplement scenarios, for inspiration when designing new encounters, to pass secret and important notes to players, and as a handy way to impart information. These decks can add new layers to plots, enhance the story, as well as [sic] introduce friendly or villainous non-player characters. Other decks in the series include: The Phobia Deck, The Unfortunate Events Deck, and The Weapons And Artifacts Deck.