Short answer
An ISBN (International Standard Book Number) is a unique identifier assigned to a book edition. Retailers and libraries use it to catalog and sell your book.
Direct answer
An ISBN (International Standard Book Number) is a unique identifier assigned to a book edition. Retailers and libraries use it to catalog and sell your book.
What authors usually misunderstand
Authors sometimes think an ISBN is the same as copyright, or that one ISBN covers all formats forever.
FloodGate's approach
FloodGate sets up ISBN and metadata as part of production packages. Who owns the ISBN depends on your publishing path.
What this means for you
Each format (ebook, paperback, hardcover) typically needs its own ISBN. Know who controls yours.