A preface or prologue is a sort of pretext to the writing that follows it; it sets up the reader, as it were, to encounter the text that it introduces. It attempts to mediate then, between the world of the reader and the world of the text by making clear an intention. Now this intention may or may not be the intention of the author, after all, especially since a preface may be written by anyone: editor, publisher, famous writer, or even the original author, not fifty years older. This mediation is not without problems: it involves a reader posited by the author of the preface who is not necessarily the actual reader; in other words, what the preface really offers is a mediation between its own time, place, and context and the text that follows it. This preface, and all the matter that shares the problematic status of “almost” text, and that is not “the” text, but rather surrounds the text, is the “paratext.”
In other words, paratext is the liminal matter that forms the bridge between the context of a text and the text itself. Following the definition of Gerard Genette in his book Seuils, the paratext lies between the text and the hors-texts, “out ce par quoi un texte se fait livre et se propose comme tel à ses lecteurs, et plus généralement au public” [all the ways in which a text makes itself a book and presents itself as such to its readers, and more generally to the public]: title, dedication, epigraph, etc.
See: A Pretext for Writing.