Backstory refers to events or things that happen to the main character (or the story world) before the story starts. While they are important to the author, they are not as important to the reader. The author needs to know the backstory, and can even write one up in order to get a better understanding of their characters. However to put all of that into the story can be a big mistake.

Time spent talking about things that happened in the past means time spent away from the current plot. Readers tend to have short memories, and they may forget what was going on in the main story if too much time is spent in the past. I'm not talking about time travel here. I'm talking about memories, flashbacks, and narrative exposition.

When backstory sounds like a history lesson, you can be sure the reader is going to forget most of it. Unless the information is directly important to understanding the plot, it's best to leave it out. Because let's face it, it's just plain boring.

Without a doubt, some backstory is important. What's also important is that it isn't presented in huge chunks, or all crammed at the beginning of the story. This goes back to short reader memories. Even if something is important to know later, the reader may not remember it when the time comes for it to be important. This is why pieces of backstory should be sprinkled throughout the story, closer to when the information needs to be known. Smaller chunks are easier to digest, and proper placement ensures its relevance in the reader's mind.

Prologues are tricky. They are often used as an excuse to feed the reader an entire chapter of backstory. If you feel tempted to write one in order to present your fantasy world to the reader, resist. It is a mistake, because it will seem like a history lesson, and easily forgotten. It is much better to reveal your world during the course of the story, bit by bit.

Proper prologues generally involve a major event in the past. One that sparks a sequence of events that leads up to the inciting event of the present story. In the Lord of the Rings movie, the prologue is the narrated portion explaining all the rings of power, and how the One ring got lost. Without this prologue the viewer may not understand the importance of the ring that is given to Frodo later.

When considering whether to include a piece of backstory, ask yourself this question: Is it needed for the reader to understand the plot or characters? Sometimes backstory is a form of telling instead of showing. For example, "Grakilot was a land of strict rules and harsh slavery, where soldiers auctioned off their prisoners of war."

Instead of saying all this as backstory, it could be demonstrated during the course of the story itself. Perhaps like this: "She wandered the orderly streets of Grakilot, carefully side-stepping the soldier yanking along a chain of prisoners behind him. 'Keep moving,' the soldier barked. 'You'll be fetching me a good price later.' The prisoners looked like tattered and bloody versions of the soldier himself. Prisoners of war, perhaps?"

Which version is more interesting? The reader gets the same information about this city, but one seems more real while the other seems more like a textbook explanation. Read some well-edited stories with this in mind and you'll start to notice that the backstory is carefully woven into the main story itself. When done well, the reader will barely notice!