eCPM (Effective CPM) is a very important parameter of Google Adsense, according to this parameter, you can analyze the Google AdSense on your website advertising effect, and to optimize and adjust the advertising targeted, in order to improve the income.
so what is eCPM? AdSense in the help document is so explained:
effective display cost per thousand (Effective CPM) – cost per one thousand shows. From the publisher’s point of view, the cost per thousand shows is a very useful way to compare the revenue of different channels and advertising programs. It is calculated by dividing the total income by one thousand times. For example, if the publisher’s display number is 45000, the income obtained is US$180, the cost per thousand display is equal to US$180/45, or US$4.00.