Header bidding, in one story

Old waterfall: the ad server rings one network at a time, and the first yes wins, even if a higher bidder sits further down the list.

Header bidding: before the ad server call, a wrapper like Prebid asks many bidders at once, inside a timeout. Bids enter the ad server as key-values and compete on price.

Shared plumbing. Your data and decisions are the special sauce.