As noted by gcarden498, "in medias res" is a Latin phrase meaning "in the middle of things." With respect to Homer's Iliad, because other poems in the Greek epic cycle dealt with the events leading up to the Trojan War, as well as the first nine year of the war, Homer's Iliad picks up in the tenth year of the war with the quarrel between Agamemnon and Achilles.
The first nine years of the Trojan War, which are not dealt with in Homer's Iliad, were relatively quiet because the Trojans learned during the early days of the war that they were no match for Achilles in open battle. Therefore, the Trojans withdrew within their fortifications and endured a nine-year seige by the Greeks. The quarrel between Achilles and Agamemnon in the war's tenth year causes Achilles' withdrawal from battle. This encourages the Trojans to come out from the safety of their fortifications and fight in the open battlefield.
We should also note that, with respect to "in medias res," Homer's Iliad never deals with the death of Achilles or the fall of Troy. These events were treated in other poems within the epic cycle. If we count up the references to the passing of days in the Iliad, we discover that Homer's Iliad covers only about a four week period in the tenth year of the war.
Homer's Odyssey is probably a better example of "in medias res" as some of Odysseus' most famous adventures are recalled by Odysseus in Books 9-12 of this 24-book epic.