Most likely, if you’re a writer you’re already familiar with what an ISBN actually is. It stands for International Standard Book Number, and is the way books are commercially set apart from each other. The book number is unique for every book, and for every version of a book, like a literary fingerprint. It allows a book to be identified both in catalogs and bookstores, be they physical or digital.
Most people know that a book needs an ISBN, but what they may not understand is that each format of the book needs a separate number. A hardback is different from a paperback, which is different from an epub, which is different from a mobi. An ISBN will reveal exactly what you’re getting, as opposed to other versions available.
If you have a publishing company, they will most likely take care of the process of assigning ISBNs to your work, but a self-published author needs to do that themselves. It’s not a difficult process, but it is an important one.