What Exactly Was Original Sin?

There are many Christians who concede that the Book of Genesis was metaphorical not literal, but I've never heard any Christian explain exactly what this metaphor represents, specifically the whole business with the apple and the expulsion from Eden. Presumably, since they still believe in original sin, there was some kind of event that got god angry at us for which Jesus needed to be sacrificed in order to save us all from hell, but what was this? What does the metaphor of the apple represent? If this was not a historical account, then what event in history actually did result in this original sin?