Header bidding is an advanced method of programmatic ad buying that allows buyers — pre-approved by the publisher to participate in the programmatic auction — to bid for impressions simultaneously.
Usually, ad servers allocate inventory in a sequential waterfall system that flows through ad exchanges, one after another. In header bidding, code is placed on a web page that loads a wrapper when the page loads. The wrapper is a container or framework within which the individual codes load for each header bidding partner. When the wrapper loads, it triggers a bid request to each of those partners, allowing them all to bid at the same time.
Publishers can maximise revenue by offering inventory to multiple buyers at once, and advertisers get an equal ‘first look’ at premium inventory. Also known as advance bidding or pre-bidding.