That's the earnings per 1,000 adsense impressions So, if you get 1,000 people to look at your pages you should earn $170 - but you won't, as such a high eCPM is an indicator of not having enough traffic / clicks yet for google to calculate the true value of your site to the adwords advertisers
ecpm is earning cost per 1000 impression. You have a vert high ecpm that means that your niche is very good. I just got around $2 ecpm
Some of my sites have eCPM of thousands of dollars - but that is only because they have limited visitors and only a few clicks here and there Once these young sites are trusted by google and start getting good traffic and regular clicks of 10 or more per day, the eCPM will drop to less than $25 - maybe less than $10 - and that's for high paying niches like finance, insurance, etc. $2 eCPM is more than most people are getting - but if you are planning on making adsense your living, then you need to find a way to build sites with much better eCPM, as there is a limit on how many visitors you can get per day using free traffic from google - and that's the only traffic to get for high adsense earnings
My highest eCPM on any my sites since my last payment is £317.96. This is for a financial site. In fact my three highest eCPM values are all for financial sites. Go figure.