What is header bidding?
Header bidding is a technique that lets publishers offer each ad impression to many demand sources at the same time, before calling their ad server, instead of offering it to one partner after another. Running bidders in parallel increases competition and usually raises publisher revenue. Prebid is the most widely used open-source header bidding software.
Before header bidding, publishers used a waterfall: the ad server tried partners in a fixed order, and the first to accept a price got the impression even if a later partner would have paid more. Google's own exchange also had advantages such as last look. Header bidding code, originally placed in the page's HTML header, collects bids from several SSPs at once and passes the best to the publisher ad server, which compares them with direct deals.
The trade-off is latency: more bidders means slower pages. Server-side header bidding moves the auction to a server to reduce browser load. The approach spread quickly after 2015, and Prebid became the open standard.