Epigraph

Epigraph: A quotation or aphorism at the beginning of a literary work that is suggestive of the theme.

Example:
"By far the larger part of the slaves know as little of their ages as horses know of theirs, and it is the wish of most masters within my knowledge to keep their slaves thus ignorant."
                      (Frederick Douglass, Pg.1)

This is an example of epigraph because by reading the first sentence we know that the main theme of the story is the harshness of slavery. We know this because he says the "wish of most masters" to keep them ignorant. Which the impact of ignorance coudl be aother theme.