Header bidding is a programmatic advertising technique that allows publishers to offer their ad inventory to multiple ad exchanges simultaneously, before making a call to their ad server. This allows publishers to receive bids from a variety of demand sources, including ad networks, DSPs, and direct advertisers, in real-time. By creating a competitive bidding environment, header bidding helps publishers maximize their ad revenue and provides advertisers with more opportunities to reach their target audience. Unlike traditional waterfall auctions, where ad exchanges are prioritized based on historical performance, header bidding allows all demand sources to compete on an equal playing field, resulting in higher CPMs and increased transparency in the ad buying process.