Roughly, paratext is the set of materials that surround the main text. The term originally referred to books, and included things like the cover, the foreword or afterword, footnotes, etc.

Paratexts also exist for things other than books. A movie trailer or poster counts. So do the director’s commentary on a DVD and all the publicity interviews given by the people affiliated with a film. So do box sets of television shows.

Paratexts don’t need to be official, either. Fan forums and recaps are paratexts, as are things like maps of Marvel’s NYC or charts showing which heroes have interacted.