That's the average you are making per 1000 impressions (i.e. for every 1000 times an Adsense ad shows on your site, you're making $1.34)
eCPM is effective Cost Per Mille. See http://adsense.blogspot.com/2006/02/ecpm-what-exactly-is-that.html for more info.
https://www.google.com/adsense/supp...=32865&ctx=en:search&query=eCPM&topic=&type=f The help is always a good thing!