What is the explanation of an epigraph in reference to literature?

In literature, an epigraph is an introductory quote taken
from one piece of literature and used at the beginning of another piece of literature.
The purpose of the epigraph is to illuminate, in one way or another, the objective or
meaning or mood etc of the piece of literature in which it is quoted. For instance, a
novel about choosing virtuous behavior in the face of great unmet need might be
introduced with this epigraph quotation from Plato: "All the gold which is under or upon
the earth is not enough to give in exchange for virtue."

An
epigraph may perform a number of functions that illuminate a text. (1) It may set-up
intertextual comparisons or contrasts. For instance, hypothetically speaking,
Tales of King Arthur may be prefaced with an epigraph from
Machiavelli's The Prince to establish a contrast between two
philosophies in the reader's mind. (2) It may serve as a preface to the chapter or story
or novel by establishing the purpose or scope the upcoming text. For instance, a chapter
on apartheid might be prefaced by this epigraph from Deuteronomy 32:28: "For they are a
nation void of counsel, / And there is no understanding in
them."

(3) An epigraph may act as a summary of the upcoming
text. For instance, a novel about a talented and successful young woman whose failing
was being self-centered and unperceptive might be summarized by this epigraph from Jane
Austen's Emma: "The real evils, indeed, of Emma's situation were
the power of having rather too much her own way, and a disposition to think a little too
well of herself; these were the disadvantages which threatened alloy to her many
enjoyments." (4) It may provide a contrasting, countering perspective to the upcoming
text. For instance, an essay against free-trade agreements may open with this
contrastive quote from John Adams' Wealth of Nations: "To widen the
market and to narrow the competition, is always the interest of the
dealers…"