An ISBN is a number. Just a number.
At the moment, all new ISBNs are a 13-digit number. You may see it formatted with spaces or hyphens between some of the digits, but the ISBN number is really just the thirteen numeric digits. It identifies a particular edition of a book (or perhaps some other book-like publication). That's its job - to uniquely identify a particular edition of a particular work of writing. It might be a paperback edition of a particular novel. Perhaps it's an edition of a hardback academic textbook, or it could even be the Amazon Kindle version of the same textbook. Each will have its own ISBN of thirteen numeric digits.
For humans in most parts of the world the easiest way to represent and read thirteen numeric digits is as they are represented using the normal set of arabic numerals of zero to nine. But that isn't the only way to represent any particular number. A common way to represent numbers that is useful for inputting to a computer or some other machine rather than be read by a human is the system of barcodes that has been developed.
The barcode and number on the back cover of the book mentioned previously is the book's ISBN represented in machine readable form and in human readable form respectively. The human readable form is the set of thirteen arabic numerals, and the computer readable (or machine readable as it's usually called) form is the barcode. So the barcode isn't the ISBN as such, but it is a representation of the ISBN that it encodes, in the same way that the arabic numerals aren't the ISBN really, but a particular representation of the number.
If you were wondering what comes first, the number or the barcode, then it should be plain by now. You can't get the barcode until you've got the ISBN. The design of the barcode follows on from the ISBN number allocated to the publication, whether it's represented by a barcode or by arabic numerals. If you want to find out how to get an ISBN for your book, then see the How To Get An ISBN Number page on this site.
If you want to read more about why an ISBN is constructed the way it is, see the Anatomy of a 13-digit ISBN page on this site. If you want to know more about the ISBN barcode format and how it is part the larger universal product code system, then see the ISBN Barcode page.