Exposition is basically how an author develops the beginning of a story by “providing of some background information to the audience about the plot, characters' histories, setting, and theme” (enotes reference, Exposition (literary technique). Exposition is a very important part of any story because it creates the world in which the story takes place. This is even more important in a science fiction story like “A Sound of Thunder” because the exposition is needed to establish the premise.
In “A Sound of Thunder” the exposition introduces us to a world sometime in the future where time travel is not only possible but common. In fact, it is being used for a business venture. Consider the sign that Eckels sees at the beginning of the story:
TIME SAFARI, INC.
SAFARIS TO ANY YEAR IN THE PAST.
YOU NAME THE ANIMAL.
WE TAKE YOU THERE.
YOU SHOOT IT. (“A Sound of Thunder”)
This one sign establishes so much. We know that this is a society much like our own. At the end of the story, the spelling changes. This is how we know the future was altered, but here the spelling is the same. We also know that the technology is available to travel anywhere, put only in the past, and that hunting in the past is popular. Note that the time machine cannot go to the future.
The exposition of the story also establishes Eckels’s character and the role of his guide Travis. We learn that Eckels is somewhat hot-headed and Travis is very blunt. We also learn the rules of time travel (stay on the path) and that there was a recent election and if the other guy had won he would have been a dictator. All of that information is key to understanding the story.
Read the full text here: http://www.lasalle.edu/~didio/courses/hon462/hon462_assets/sound_of_thunder.htm