CPM means "clicks per thousand" -- in other words, the number of clicks you get per 1,000 ad impressions on your page. As far as what it means for pay, that depends on whether the articles shown are pay-per-click or pay-per-impression (AdSense shows a combination) and how much was paid for them. You can get a bit of an idea of the value of particular keywords by checking the AdWords keyword auctions and see what the current price is -- there is no definite way to know what someone paid for the ads that will show up on your page, but that can help you steer toward decent-paying keywords. I don't currently know of a hub on this, but I will be writing one and doing a video demonstrating how to optimize for good-paying AdSense ads. If I don't get to it this week, it should be up early next week, so if interested stay tuned ;).