Okay, so we're just sitting here, chowing down on some popcorn (with Milk Duds sprinkled in, of course) watching this bald guy mill around on a desert island, poking through the wreckage of Oceanic Flight 815. And then this really weird sound comes out of nowhere (Shmooooooooooop) and boom, he's in a wheelchair demanding he be allowed to go on a walkabout in Australia.
Wait. What just happened?
But flashbacks are more than just a TV trope. Books flashback quite a bit, too. Flashbacks occur when whatever you're reading breaks with chronology to give us a glimpse of the past—or the future (we call that a flashforward—hey, wasn't that another TV show? And another one?). Flashbacks can come in the form of a conversation, a dream, or a memory. Or they can just happen, out of the blue, Lost style.
You'll find flashbacks pretty helpful. While they can confuse us by messing with our sense of time, they usually give us a backstory that helps explain what's going on in the present, so we can make sense of what our characters are up to.