From the back of the box:
All manner of strange, ill-fated, and disastrous situations that can affect, hinder, and trouble your investigators. Use in game [sic] to add additional drama and upsets for your 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 Curious Characters Deck, The Phobia Deck, and The Weapons And Artifacts Deck.