It is the number of cards you created/imported, which are neither in the review nor learning process. Those are the cards which appear if you learn new cards. Maybe you create more cards than you learn, in this case, you might want to bump the amount of daily new cards up a bit.
Depending on the settings, new cards will either be introduced randomly or in the order created. You can filter in the card browser for unseen cards to see which ones they are.