ISBN stands for International Standard Book Number. You can find the ISBN for your book in several places if you are a Sapphire, Ruby, Diamond, or Pearl author (Emerald books do not receive ISBNs because they are not distributed; only books sold through distribution, wholesalers, or retailers require ISBNs).
The ISBN is on the copyright page of the book. It is also on the back cover in the barcode. It is also on your author webpage provided by Outskirts Press.