Pair of what? No, no, grandpa, now's no time for jokes. We're talking definitions here.
Paratexts are basically everything except the text—everything that's beside (para in Greek) the body of the work. So when you're reading The Great Gatsby, that awesome cover is considered a paratext. So is the table of contents of your current People magazine issue and the index of your high school science textbook.
When we think about paratexts, we think mostly about the dedication, the preface and the afterword. The dedication is so cool it gets its own entry.
The preface is a nice little ditty written by the author that comes before (pre) the work itself—not to be confused with the foreword, which isn't written by the author. The preface might talk about how the work was put together, or what awesome people helped the author work out all the kinks. Or, in the case of Moll Flanders, how Daniel DeFoe had to change up his story to make his protagonist more believable.
The afterword is kind of the same thing, but at the end (after) of the book. Like when Lois Lowry gives us some based-on-a-true-story info in the afterword of Number the Stars. Just to complicate things a bit more, the afterword can be written by someone other than the author.