Header bidding, also known as "pre-bid" or "advanced bidding," is a technique used in the advertising industry to allow multiple demand-side platforms (DSPs) to bid on ad inventory at the same time, rather than using a traditional "waterfall" model in which bids are passed from one DSP to the next.

Header bidding is called "header bidding" because the JavaScript code that implements the bidding process is placed in the "header" of a website's HTML code. The header of a website is the top portion of the code that is read by the web browser when a user accesses the website. By placing the header bidding code in the header, it is able to execute before the rest of the website's content is loaded, allowing the bids to be submitted and the ad to be served more quickly. With header bidding, all of the DSPs are able to bid on the ad inventory simultaneously, which can lead to higher ad prices and better ad performance. Header bidding is typically implemented through the use of a "header bidding wrapper." When a user visits the website, the wrapper sends out ad requests to all of the DSPs that are participating in the header bidding process, and the DSPs are able to bid on the ad inventory in real-time. The winning bid is then passed back to the ad server, which serves the ad to the user.

In a traditional waterfall model, an ad server will send an ad request to the first DSP in the chain, and if that DSP is unable to fill the request, the ad server will move on to the next DSP and so on until the ad is filled. This can lead to inefficiencies, as the ad server may have to go through multiple DSPs before finding one that is able to fill the request.

Header bidding has become increasingly popular in recent years, as it allows publishers to get higher ad prices and gives advertisers more control over the ad inventory that they are purchasing. However, it can also increase the complexity of the ad delivery process and may require more technical resources to implement.