Back story is a tricky topic. You can't live with it and your MC can't live without it! The term "Back story" encompasses all those things that may have happened to your main character (or to the townspeople he's trying to help) before we meet him in the first pages of your book.
Take the first chapter of Harry Potter and the Sorcerer's Stone**. J.K. Rowling sets the stage and the characters for her epic YA series in the first chapter and reveals a surprising amount of back story to her readers succinctly and with masterful characterization and style. By the end of chapter one we know all we need to know about the Dursley family and have tantalizing bits about Dumbledore, McGonagall, and Hagrid to carry us further into the story... as well as some crucial details about Harry Potter himself. Chapter two begins ten years later! We don't get any detail about what those ten years entailed, and we don't really need them because the characterization of Harry and the Dursleys in the subsequent scenes tells us everything, and I mean everything, we need to know just what happened during those ten years. But you can bet that J.K. Rowling knows what every minute of every day of Harry's childhood was like.
Each piece of back story that you know for your character can help create a more richly rounded picture of him (i.e., how and why he reacts to given situations, why he knows or thinks what he does about certain topics). You may think that it's just as important for your reader to know all this back story as it is for you. Honestly, though, it's not important... at least not yet.
SOME of this back story information is pertinent to the novel. MOST of it is not. Certain bits of back story, when they are revealed, dramatically enhance the action and push forward your plot. Knowing which bits are which can be difficult. Having someone read your manuscript with a critical eye and point out bits that don't pertain directly to the plot can help a lot!
Don't throw out or summarily delete all those delectable details, though! Someday you will have fans who will devour those bits. Save them for the special edition you release to your adoring fans after you've hit the bestseller list! Which bestseller list am I talking about? That is up to you.
**Editorial confession... I love Harry Potter deeply and dearly. I will likely use examples from that series a considerable amount during this series. If you love Harry Potter, too, I think we're meant to be BFFs for life. If you DON'T like Harry Potter... I'm sorry but it's just not going to work out between us...