In open ad exchanges, all advertisers have an equal opportunity to buy (either manually or programmatically) available ad inventory from publishers via an open auction.
This contrasts with private marketplaces where publishers control who can access the exchange and participate in the auction.
Publishers use open ad exchanges to sell remnant ad inventory at a low cost. There is no pre-agreed relationship between either party, and publishers can choose to withhold information, so the advertiser may not know which specific placements they are buying.
For publishers, the low prices may be offset by advantages such as a large pool of buyers, the potential for high-volume sales, and the opportunity to generate revenue from inventory that may otherwise go unsold.
See also, Private Marketplace.