In medias res is a Latin expression that means "in the middle of things." It's often used in literature to describe plunging straight into a dramatically important situation in the middle of a story instead of starting right at the very beginning. So in The Odyssey, for example, the action doesn't begin after the fall of Troy, but with Odysseus languishing on the island of Ogygia, where he's spend the last seven years detained by the nymph Calypso.
Homer's use of the technique is effective because it immediately grabs our attention. We want to find out just how Odysseus came to be on the island and what his next moves are likely to be. By keeping us in suspense like this, Homer holds our interest and creates a narrative space for the subsequent recounting of the first part of Odysseus's epic journey.