Most stories have 5 stages: exposition, rising action, climax, falling action, and resolution (denouement). Although most exposition occurs in the first stage, it is also used to introduce new scenes and new characters throughout the story. It’s the explanation of what happened before the current story begins (backstory), the details about the time period and location that comprise the setting, and the description of the characters. It puts the current story in context so the reader understands what is going on, why things happen the way they do, and what motivates the characters. It can reveal the theme of the story, set the mood, and explain the plot.
Much of the exposition is done through narration, but long sections of it tend to slow the pace of the story and may be viewed as an information dump. Keep exposition to a minimum, providing just the essential information. Avoid mentioning anything that doesn’t advance the plot, help the reader visualize the characters or setting, or set the tone. Instead of giving long explanations at the beginning of a scene, try to incorporate the important details into the action, or have them expressed through the characters’ dialog or interior monologues. Even though exposition is necessary to explain the story, it is not a part of the story itself and should not overshadow the plot.