For years, publishers were at the mercy of the waterfall. When partners couldn't deliver an impression, the next partner in line would get their turn to serve. This meant a lower CPM and more latency for you. A death trap for publishers.
Header bidding interrupted the space with a first look at inventory to drive up your revenue and stop publisher from throwing ad dollars into a black box ad server.
The header bidder grabs a bid from its own server and returns with a bid before your ad server fires. When your ad server does fire, the bid is injected into the auction so that it may participate with other partners, direct sales, and programmatic.