Backstory is the history or what happened to your characters in the past. When I listen to my critique group's discussion on all the manuscripts, they comment frequently that writers often include too much of the past in their mysteries. It's hard if you write a series. Not all of your readers will start with the first book and read in the order that they were published. You have to have some linkage to the previous works. And if key parts of the story affect your characters' lives, then you need to mention it.
The key is to sprinkle in the backstory. It should be a line or two - not paragraphs. When you do the latter, it causes the reader to move away from the story's momentum. If the plot jumps back in time for an extended period, it breaks the action and the forward progress. You will lose people if there is too much historical detail.
I like to know all about my characters, and sometimes, I include too much in my drafts. To help me avoid this, I created a document. It's nothing fancy. It's a table in Microsoft Word with a column for each book. I list each character and where they appear. At the character's first appearance, I include a short biography where I list everything that I can possibly think of (e.g. favorite colors, middle names, colleges, etc.). Most of this stuff never makes it to the book, but I have it in case I need to add it for some reason. I found that by writing these mini-bios, it cures me of wanting to overly describe each character. It also helps me keep the details like color of someone's car, eye color, and wardrobe preferences straight. I also have entries in my table for the key settings in the stories. I've described furniture in my sleuth's home and office, and I want to make sure that they are consistent from book to book.
History and details are good when used sparingly. Sprinkle them in, and it will help round out your characters. Happy writing!