When a writer interrupts her main narrative to fill the reader in on a pertinent detail or two about the back story or a character’s history, and then resumes the main narrative, that interruption is exposition. It’s sometimes referred to as an information dump or info-dump, as that’s exactly what it is: a dump of information into an otherwise-continual text. It’s that Scooby-Doo moment that you see in so many TV shows and films where the bad guy pauses once he’s got his hands on the good guy in order to explain exactly why he’s trying to destroy mankind, when you know all the time that a real bad guy would just shut up and shoot.
I’m going to tell you a story here about how I nearly throttled one of my cats just now because of the terrible thing that she just did, but first you have to know that this particular cat came from a sanctuary. She’d been rescued from a neglectful owner and was hit by a car and now has one leg shorter than the others, but she’s gorgeous and I adore her even though she loves my husband and despises me.
And now who remembers or even cares that I was about to throttle her?*
All that extraneous stuff about my cat—the sanctuary, the car accident, her funny short leg and her over-close relationship with my husband—is all exposition. By revealing things in this way you make your readers lose focus on the story and when that happens too often or too harshly, they’re going to lose interest too.
Info-dumps can often just be deleted without leaving a significant hole in the text: it’s amazing how much information can be imparted through implication. If you really need to include the information then you have to incorporate it more smoothly into the text, to avoid those sudden dislocations as you switch your readers’ attention into and out of the info-dumps. This usually means that you’ll have to relocate the information to another more natural place in the text, and you’ll have to rewrite it so that the information it contains is incorporated more tightly into the characterisation, setting or plot. Which might mean that you need to add a scene, or explore an existing scene more deeply than you have.
*For those of you who are still interested, it was because she jumped up on the keyboard and managed to delete half of the first draft of this post. Bless her little furry feet. She’d better not do it again.