Exposition is a broad term to define. It really depends on the context. Within literature, exposition is usually when the author offers background information. The information could be on the plot, setting, or something about the characters. In a word, the author is setting up the story, so that the reader has enough information.
Within more academic literature, the term, exposition is usually used when a scholar writes a paper to explain something difficult. There are plenty of difficult concepts and passages in literature. Expositions are used to explore and explain. Therefore, when a student takes an expository class in the English department, they are in essence taking a course in which they will dissect literature, interpret it, and finally write their opinions on it.
An exposition is the starting of the story or narrative that provides necessary background information to understand the plot of the story.