CPM = Cost Per M M is roman numeral for 1000. CPM = cost of 1000 impressions for your site. Google uses eCMP = effective CPM. eCPM for your site(or sites) is how much it would cost to display a banner 1000 times on your site. When Google adsense finds no keywords for displaying ads on your site, it displays Public service ads (like Google Map, Chrome browser etc) which earn you nothing. You can disable public service ads in your settings.