Backstory - what is it? I think of backstory as the information you get when you meet someone and learn about their life - where they went to school, how many people are in their family, what is their religion, what is their political viewpoint, etc. These little details that have shaped a person over their lifetime. The person they have become in their life depends on so many socioeconomic factors that it is hard to label all of them. If you were to learn all of these things about a person when you first meet them, you won't have anything to discuss the next time you meet - if you even decide you want to spend time with them later on. Think about this - when you meet someone - you don't tell them about your last bad relationship and yet this is an important factor in how you deal with future relationships. Would you bring this up right away in your book as well? You might not - because you don't want the reader to know everything that motivates your character. You want to give hints and then, when the time is right - you reveal it because it's necessary for the plot development (or relationship).
Unfortunately, you cannot be inside your reader's head. You do not know how much is too much or not enough. You can only go with what works for you. Do it move the plot along properly? Then it's in the right place. Does it bog down the action - then it's too much. You would not tell a new friend about Uncle Harry's battle with cancer while you're playing softball, would you? No, but if you learn your friend has a relative with cancer and you can empathize with her feelings, you might reveal it then. So, this bit of backstory is only important at a certain moment in time. Otherwise, you keep that information to yourself. The same is true in a story.
How many of you create a character sketch before you work on the story? You know everything about your character, from the size of his/her shoes, to how many aunts and uncles, to even the favorite ice cream or magazine they read. And yet, when you write the story, it may never come up that he loves to read Cosmo unless something happens in the plot to reveal it. But for your purposes, it's important information to help shape your character. All of these little details can be backstory as well, and yet they can only be revealed when necessary.
What else is backstory? Educational background. Is it important your character attend college? I read a book by Blythe Gifford In the Master's Bed where a female was disguised as a male while at Oxford when females were not allowed anywhere near the place. She was an extremely intelligent female and this educational background of hers was key in order for her to blend in with the men at this university Another instance is Diana Gabaldon's the Outlander. The heroine has a medical background. She is a nurse. When she is plunged into Georgian times in Scotland, it's important that we learn she is familiar with natural healing methods as well as her nurses' training so we can believe she knows how to use certain herbs to help the people she meets nearly two hundred years in the past. The author sprinkles this information into the book where it is needed. If we read about her special skills with herbs earlier on, we would have wondered why it was here when it had no significance to furthering the action along or in making our heroine believable. And finally, in Sherrilyn Kenyon's Darkhunter books, we never really knew anything about Acheron, but boy, oh boy do we want to know! When the book about him finally came out, it was well worth the wait. Acheron was an extremely complex individual and his backstory was so twisted and heart-wrenching, we, the readers, were willing to read a bit more backstory because by this time, we WANTED to know.
By the same token, we would not believe the hero so ready to believe she is someone from the future until we learn about his own educational background. Here is a man who's spiritual upbringing allows him to make the leap of faith that this woman tumbled through a time portal when she finally tells him the truth. Until then, she could not risk telling him this part of her backstory. No one would have believed her.
Here is another example - when I read books by my good friend, Remi Hunter, I can hear the Chicago copper lingo coming through. I know she is knowledgeable about Chicago by her comments and how her character speak. When she is explaining a procedural action, she does it in a way that does not bog down the reader with too much police jargon that I want to roll my eyes and yell out "uncle". She puts in just enough in the right places that I go, "okay" and I can move on and keep working on the story. This is not an easy thing to do, but she does it extremely well.
I have read some authors only start their stories with prologues to get some of the backstory out the way. Others always open with dialogue and wait several pages to include backstory. Every author has his/her own theory about backstory - how much and when to add it. For each author's sentiments there are many totally opposite them. Why? Because it doesn't work for them. I like to go back to my beginning - did you ever walk down the street and wonder - why is that girl with that guy? And when you're on the beach you wonder "how could they read that book? I just couldn't get into it?" There are so many different opinions out there which is why it works in many different ways. SO when you are writing your story, remember there are no hard fast rules for backstory. It all depends on your story and your characters. You will know how much is too much and when to add it or take it out. Listen to the advice of published authors and other friends you trust, but in the end, if it doesn't work for you the way they recommend, then perhaps that is not the way to do it.