it comes from CPM. It essentially means that you'd make the CPM shown if you had exactly 1000 impressions on your site.
the CPM ads are only displayed if an advertiser elected to have adds appearing on your site AND the earnings you would get from a CPM add would be greater than what you would earn for a CPC ad for a given period of time ...that is the idea at least...do u feel you are earning more or less than previously? cheers,