The Lore book is a way for you to convey knowledge to the group as they go. As a GM, each campaign starts you with an introductory place holder for the story of your campaign. As the campaign progresses, the DM has a tool that allows them to add to the lore book based on character knowledge. An example would be a conversation with an NPC like a bar tender that tells you where something is or what it does. The DM can add that knowledge to the lore book so you can reference it in the future.