Header bidding. You sign up with a number of popular bidders (OpenX, AppNexus, Rubicon, AOL, Criteo, Sovrn, etc), and they all bid against each other in real-time. You then relay that bid to AdX/AdSense, and Google attempts to outbid that offer. Then, you display the ad from the highest bidder. This happens for every single ad impression.
Overall, this usually provides a 30%+ increase in revenue over running just AdSense.
Overall, this usually provides a 30%+ increase in revenue over running just AdSense.