Header bidding is an advanced method of programmatic advertising where publishers have the opportunity to offer inventory multiple ad exchange before making a call to an ad server.
By letting multiple DSPs bd on a the same inventory made available by a specific publishers this makes the publisher to make money.
The reason why publishers like this method is because the opportunity to control the number of impressions that they are making available to the advertiser. Through the header bidding method the publisher has complete control of the inventory on a pre-impression basis and gives the idea of how much set of impressions are worth.
The biggest problem that is faced with header bidding is that the page load takes time. So the ads can take time to load.