A private marketplace is an invitation-only platform that allows advertisers to buy ad inventory directly from a publisher or group of publishers. In a private marketplace, the publisher has complete control over which advertisers are able to access their inventory and at what price.

Private marketplaces can offer a number of benefits to both advertisers and publishers. For advertisers, they can provide access to high-quality inventory that may not be available on open exchanges, as well as more transparency and control over the ad buying process. For publishers, private marketplaces can provide a way to monetize their inventory more effectively and to build deeper, more valuable relationships with their advertisers.

Private marketplaces have become increasingly popular in recent years as a way for advertisers to access high-quality inventory in a more controlled and transparent environment. However, they can also be more expensive than open exchanges, as the prices for ad inventory are typically set by the publisher rather than determined through an open auction.