In the advertising sector, the term "reservation" refers to the guaranteed purchase of ad space for a specific period. Unlike buying ad spaces through systems like real-time bidding (RTB) where advertisers bid for space in real-time, with a "reservation" the advertiser secures a specific space or a guaranteed number of impressions for a certain duration at a fixed price.
This approach is often used for premium placements or for highly significant advertising campaigns where the advertiser wants to ensure visibility in certain spaces or times, without risking losing the opportunity due to competition in a real-time auction.