If a piece of literature is serialized, it is published in installments. This isn't so common nowadays, but in ye olden times, novelists like Charles Dickens wrote serial novels, which were published, section-by-section, in magazines.
Today we see serialization in the comic book world, and also when we watch television dramas, which are serialized in the form of weekly episodes. And thank goodness. Otherwise, Lost would have been one very long movie.