Prebid is a free open source library and community that helps publishers implement Header Bidding on their websites & apps.

Header Bidding (also known as header auctions parallel bidding or header bidding) is a technique that involves running SSP & Ad Exchange code directly on page so publishers can receive bids on their inventory that may be unavailable through their primary ad server and exchange.

The returned bids are then passed into the ad server so they can compete with direct demand and the primary ad server’s exchange on a level playing field.