What is an Inventory?
Inventory is the amount of ad space (or the number of advertisements) that a publisher has available to sell. While the term originated from print, it has grown to encompass ad space on the web and on apps and mobile ads.
Why is an Inventory Important?
Inventories are important for publishers because the more inventory a publisher has, the more they can sell. For advertisers, it’s important to find the best space and the right type of inventory at a cost which is optimal for the campaign
Reference: “Adjust,” Retrieved – 23 April 2018