Guaranteed inventory refers to the part of ad inventory which is "traditionaly" sold to agencies or advertisers by in house direct sales teams or through premium networks.
With guaranteed inventory, the publisher or network is commited for impression volume, placement and delivery time as it is specified in the insertion order.
Guaranteed inventory is typically sold according to ratecard or media kit prices and terms and after back and forth and negociations.
Guaranteed inventory is more expensive than non-guaranteed inventory but it gives better controls (volume, budget, time frame, placement,..) for the advertiser.
For the most part, premium inventory is sold as guaranteed inventory.