In medias res (Latin: in the middle of things) is the literary term for beginning a story by jumping right in to a defining moment that relates to past and future events -- all of which will be unfolded gradually through flashbacks, dialogue, and narrative.
Homer did it by beginning The Odyssey with Odysseus in captivity, his journey already well underway. Dante's Divine Comedy begins with the main character in the middle of his life when he must make a crucial choice. Shakespeare opens "Hamlet" with the king's ghost and then fills in the important back story of the murder.
It's the beginning of another semester of writing class and once again, I'm finding novel openings that drag because the writer feels the need of filling in all the back story before the action starts. It seems to be the most common mistake that new writers make.
And once again, as I always do, I'll use the analogy of meeting a stranger at a party. You don't need to hear all about their background to find them interesting -- on the contrary, unless they were raised by wolves or something equally odd, you don't need their background at all. You are far more likely to be interested in what they're doing now--whether it be raising miniature horses or running a Pastafarian music ministry for hospice. The background details can get filled in later.
And again, I'l remind them, that the opening sentence, paragraph, page may be as far as a potential agent, overloaded contest judge, or busy editor will read. It doesn't matter if page three contains the most spectacular and beautifully written scene in the history of literature --if the reader has already decided this isn't for him.