Unless you've decided to model your books after Charles Dickens, there's a good chance that many of your characters don't come to your book fresh and new. Some characters have a past, previous events that have shaped them...events that may continue to influence them. When this past becomes relevant to the character's present or future, the reader may need to know more about the past. The easiest and best way to do it? Flashbacks. But if you use them, use them with care...because flashbacks get very annoying very quickly.
The Past...Within the Past
By and large, fictional tales are told in the past tense. Flashbacks are always events that have happened in the past. So when you start throwing flashbacks into books that are already relating a story that has already happened...well, it gets confusing. This is only one of the reasons why you have to use extreme care when you write flashacks into your tale.
The other reason? It gets old really, really quickly. If I'm moving through a story, I want to stay in the timeline with the character. I want to see what develops next, and most of the time I don't really care about all the stuff that has transpired before. If those past events were meant to be the story, wouldn't they be the story? Anything that disrupts the flow of the story you're telling can make readers put the book down, maybe for good. So if you're going to go back into the deep past, you better have a great reason for doing so.
Why Flash Back?
However, the flashback is an oft-used literary technique. It's absolutely the best way to present a piece of the story that happened long ago, and show how it remains relevant to the story that's occurring in the book now. It's much better to put the reader into that past moment, and show them those events, rather than to simply refer to those events (telling the reader about them instead).
But do it too much, and you're just telling the wrong story. Readers want to stay in the action and in the flow of the narrative, and that's where you've got to keep them. So use flashbacks to make very specific points and provide very important information, but that's it. They are not meant to be filler and they shouldn't be over-done, because at the end of the chapter the reader doesn't want to know the character's entire life history. They just want the story, so don't use too many elements that will interrupt it.