Serial fiction is a story released in instalments rather than as a finished book. The form is old, Dickens worked in it, and the reading apps have rebuilt it around a payment screen: chapters are short, they arrive on a schedule, and most platforms charge for them one at a time.
How the apps use it
A serial app typically gives away the first several chapters, then asks for coins. Chapters run a few hundred to a couple of thousand words, which is short enough that a reader finishes one in a few minutes and long enough to end on a question. That rhythm is the product.
It changes the writing. Each chapter needs a hook at the end, so plots are built from near misses and interruptions. Characters repeat what they want, because a reader may be returning a day later. Nothing is resolved early, because resolution is what you are paying for.
What it costs
Almost never a flat price. You buy coins, chapters cost coins, and the coin packs are priced so you always have some left over. Working out the real cost means converting a pack price into a per-chapter price and multiplying by the length of the book, which can run to hundreds of chapters.
Serial, serialised, and finished
A serialised book is one that was written as a book and is being released in pieces. A true serial is written as it goes, and may never end: ongoing stories can run for years, and some are abandoned mid-arc. If you mind that, check whether a title is marked complete before you start buying chapters.
See also cliffhanger paywall, which is the technique serials are built on, and the glossary.