In the context of online advertising, "inventory" refers to the available ad space on a website or app. Advertisers can purchase this ad space in order to display their ads to the users of the website or app. The amount of inventory that is available can vary widely depending on the size and popularity of the website or app.

Publishers, or the owners of the website or app, are responsible for managing their inventory and determining which ads will be displayed. They may use various methods to do this, such as working with an ad server or using a demand-side platform (DSP) to sell their inventory to advertisers.

In order to attract advertisers, it is important for publishers to have a large amount of high-quality inventory that is well-targeted to the interests of their users. Advertisers, on the other hand, want to purchase inventory that will reach their target audience and drive the desired results for their ad campaign.