CPM stands for "cost per 1000 impressions." Advertisers running CPM ads set their desired price per 1000 ads served and pay each time their ad appears.
As a publisher, you'll earn revenue each time a CPM ad is served to your page and viewed by a user. CPM ads compete against cost per click (CPC) ads in our ad auction, and we'll display whichever ad is expected to earn more revenue for you.