The Keeper of Arcane Lore is the title for the person who runs the game in Call of Cthulhu, the counterpart to D&D’s Dungeon Master. The term "Keeper" is used consistently throughout Chaosium’s rulebooks.
As the name says, the Keeper is the custodian of arcane knowledge. Holding Mythos secrets the Investigators do not yet know, the Keeper paces when and how those truths surface, guiding the cosmic-horror scenario.