When the book you're reading begins in medias res, it starts right in the middle of the action, often filling in background information using dialogue and flashbacks.
Beginning in medias res means you're thrust right into the heart of the story — the most interesting and action-packed part. Movies and books that start this way use various techniques to gradually fill in the exposition, so viewers or readers don't feel like they've missed something important. The phrase ("into the middle of things" in Latin) was coined by the Roman poet Horace, who said the ideal poet "hurries to the action, and snatches the listener in medias res." Many epic poems, including The Odyssey and The Iliad being in medias res.