It is known by the term prologue to that brief writing that will always be found at the beginning of an extensive literary work, among what are called preliminary documents and that is generally used by the author of the same to explain to the readers the motivations that led to create it or to highlight some aspects that they consider decisive and relevant when reading the piece, although it will also help the latter to orient themselves a bit in reading it, because prologues often offer the reader the keys to the interpretation of the work.
However, these are not the only functions of a prologue and it will not always be the author of the prologue who writes the prologue in his own hand, but we can cite some other possibilities such as: carry out a literary criticism of the author, introduce him to the public the work of an unknown author, orient the reader about the modifications that a work has undergone, among them, extensions, deletions, updates, the theoretical framework used, as a thank you to remember all those who participated and made the work possible and defend a work and explain its merit.
When I pointed out above that sometimes the author of a work may not be the same as the one who wrote the prologue, it is because, for example. It may be a question of a very new author, little known, then, the resource of a prologue written by a recognized pen that endorses and weighs the work of the new author will be used.
The prologue will never be written before finishing writing the work, but its writing will be later than this to give rise to what we discussed before, because without a finished work, of course, an author would have little to say about it.
Meanwhile, if it is a very successful work that has several editions, it is common that for each reissue a new prologue is prepared, which will surely bring up something of this question of success.
Although sometimes people tend to confuse them, a prologue differs from a simple introduction because it lacks the literary character that most prologues display
