The International Standard Book Number (ISBN) is a number that identifies pertinent information about your book: Country of publication, publisher, title, etc. It’s used internationally for a quick way to look up a particular book and, while you don’t need one for a book that isn’t published or that you are only selling yourself directly (without going through stores), you will need them for any book that is widely published. You can find a lot more detailed information on the ISBN.org website, but here is a quick and dirty break-down:
Every book and book format needs an ISBN. While some platforms or stores may make this an optional field during the publishing process, most stores and distributors require it. You need a unique number for each book in each format or binding you publish. In other words, you will need separate ISBNs for your eBook, paperback, hardcover, and audio version of every translation of every single title. That identification number corresponds to that edition of that binding/format of that book. If you make any changes to the book, it constitutes a new edition and you will need a new ISBN number.
Some platforms offer a free version or native alternative. You can request a free ISBN if you publish through Smashwords or CreateSpace, for example. The free ISBN designates that platform as the publisher of your book. It’s an option if you want to keep things simple and don’t particularly care about the details of what that number actually means. Amazon KDP does not require an ISBN number. It does, however, automatically assign an ASIN number, which is similar, but only applicable to Amazon’s Kindle store. Since the KDP publishing platform is Amazon-specific and won’t let you distribute outside of that, an ISBN is not necessary, but if you have one, put it in.
ISBNs are not assigned automatically. They need to be requested, and purchased, and once they are purchased you need to report back to the agency where you purchased it when the numbers have been assigned and where. Prices will be listed on the agency’s website and you will get a discount for bulk orders, which they recommend you get so you can control the publisher number.
Obviously, this is not something traditionally published authors will need to worry about. I personally don’t consider it a big issue for self-published authors, either–I usually try to make things as easy on my self as I can–but if you are super worried about it, you now know your options.