The exposition is the part of a book that sets the stage for the drama to follow: it introduces the theme, setting, characters, and circumstances at the story’s beginnings. To identify the exposition, find in the first few chapters (or pages) where the author gives a description of the setting and the mood before the action takes place.
In the story of The Three Little Bears, the exposition goes something like this:
Once upon a time there were three little bears, a mama bear, a papa bear, and a baby bear. They lived deep in the forest, far away from any people. One morning, mama bear decided to treat her family to a hot serving of porridge.
See how this sets the stage for the action to come? You just know that these bears have lived a pretty ordinary life until now—but something extraordinary is about to take place.