Ad Inventory (or Advertising Inventory) is the ad space that a publisher is selling, which advertisers can buy. This ad space can be print or digital - in websites, videos, mobile apps, mobile games, and more.
The value of ad inventory is determined by the traffic it creates (e.g. how many impressions or views it can generate). The homepage of an app or game, for example, would be considered high-value ad inventory due to its large customer exposure. Nowadays, ad inventory is often sold programmatically, through ad exchanges or real-time bidding.