Every story must convey information. The author must unveil the story world. The reader will learn about the setting, the characters in that setting, and what the characters do. The question is how to present this information. The author could tell it. For example, Jack entered the bedroom. He wanted to kill his wife, Jill. Or the author could show it. Jack entered the bedroom. He quietly approached the bed, where his wife was sleeping. The shadow of his knife fell across Jill’s neck. In the latter, Jack’s motivation is implied in his actions. It’s shown, not expressly told. A basic principle of good writing is to show, not tell. Exposition is telling. Exposition, therefore, is discouraged.
Nevertheless, there are occasions in storytelling when showing is simply impractical. Imagine that Jack’s bedroom is on a space-station 100 years from now. A whole sequence of cosmic events caused the sun to explode, blowing Earth to bits, and now that station orbits a remaining chunk of destroyed planet. Suppose the author wants to introduce this backstory at the beginning of her story. Creating scenes which show all these events might be very clumsy, adding too many scenes before we get to the scenes about Jack, our main character. However, simply telling the reader the history is exposition.
An effective compromise is disguised exposition. One character tells another character the information. The reader hears the conversation, thereby learning the information. But there are rules here, which damage a story if violated. Rule 1: character A must tell character B something B doesn’t already know. Rule 2: character A must tell it to B in a context or situation where B would want this information. Violate either of these principles, and your dialogue is unrealistic.
As for Rule 1, you will not tell your dad that he is a male. Why? He already knows this fact. People don’t iterate to others what others already know. People share new pieces of information. As for Rule 2, you will not tell your dad to watch his step and not slip on your bathroom floor, while the two of you are making a campfire 500 miles away from your house. Camping with your father is simply not a context or situation where your dad would want this information (because it’s not relevant).
Disguised exposition can delivered via mass media in a story: e.g., newscaster, newspaper, talk show, documentary, news interviews, magazine article, or book entry. In each case, a character in the story world is telling another character information. (We may not be introduced to all these characters, but the speaker and the recipient are still characters in that world.) Exposition is not as obvious using these methods, because it doesn’t violate the two rules above. Rule 1 is not violated, because a newscaster is not addressing any particular person. Although one character might know the information, many other characters won’t. Rule 2 is not violated, because a newscaster is not addressing any particular viewer’s situation. The newscaster is not sharing a particular situation with a particular viewer (such as lunch, or running away from the villain, or building a campfire).
In the example of Jack on the space station, the backstory could be told very quickly by cutting from one newscast to another. Those newscasts could skip in time, each providing a piece of the backstory. The reader is given a brief montage of newscasts, which is likely far more engaging than a preface of exposition. Naturally, mass media can be used at any place in the story where exposition is needed, not just the beginning.