In general, non-guaranteed line items fill your site's unsold inventory when you haven't made any contractual impression agreements with your advertisers. Non-guaranteed line items include: network, bulk, price priority, and house. They aren't considered booked inventory by forecasting.
If you are a content producer or a freemium app or game developer, you would know, instantly, what I am talking about: there is WAY too much ad inventory out there. Too many eyeballs that are not getting adequately monetized. Major publishers sitting on top of huge masses of unmonetized impressions. Game developers monetizing, barely, 1-2% of their traffic. App developers, similarly, struggling to convert free users to premium.