Header bidding means that publishers can offer ad space to numerous supply-side platforms (SSP) or ad exchanges at the same time. Before that, there was a waterfall system, ad space was sold to a single advertiser and only their ad was shown until their frequency ran out. In a waterfall system, the priority was given to the biggest publisher, the highest bidder has a priority in header bidding. At the same time, publishers themselves may choose an SSP.
Header bidding offers publishers control over their advertisers, an increase in pricing, the number of advertisers, and the quality of ads. It also helps to load pages faster since it decreases the amount of time needed to conduct an auction for ad space.