I would like to know what does website inventory mean? So for example, if someone say they want to buy my US inventory, what does that actually mean? Does it mean buying the content or the website? Or what?
Like Sarah said, it's your banner space. More specifically, it's the banner space that reaches visitors from the United States only. Since the web is boundary free, you get visitors from all over the world. In doing so, you can specifically isolate your US viewers to see that ad only. This will allow you to reserve your outside of US visitors for other advertising campaigns to increase revenue.