It stands for International Standard Book Number. It is a 13-digit international registration number used by the entire publishing industry to identify, classify, and market, both, physical and digital books. The ISBN grants you international registration, and no one else will have an identical number to confuse it with. As a benefit, this indexed number can be used as a search engine on the net, and in physical and digital stores.
It is not mandatory to have an ISBN, but it is important to emphasize that if you already have one for a paper edition of your book, it is not useful for the digital edition. You must process a different one. If you want to process it, visit the International ISBN Agency to find the agency responsible for issuing ISBNs in your country.