"In media res" is Latin meaning "in the middle/midst of things." In literature, stories which begin in the middle of something, rather than in the beginning, are referred to as beginning in media res. Typical of both Anglo-Saxon and Medieval (Arthurian) texts, literature which begins in media res places the reader into something which is already happening. The introduction is unlike a typical introduction--the reader is not slowly introduced to the setting and characters. Instead, the reader is placed into the action which has already been set in motion.
Erenst Hemingway's "The Short Happy Life of Francis Macomber" begins in media res. This initial sentence states to readers that something has already happened which places the characters where they currently are. "It was now lunch time and they were all sitting under the double green fly of the dining tent pretending that nothing had happened." Whatever happened to force the characters to act as if nothing did happen exists prior to the opening of the story. This is defined as in media res.