Non-technically, a CNAME DNS record says "you can call me this, but treat me like that".  In its benign uses, it enables things like custom domains on hosts; [Github Pages](https://pages.github.com/) provides me with the domain pes10k.github.io, but with a CNAME instruction visitors can use [www.peteresnyder.com](http://www.peteresnyder.com/) to reach my github pages site, regardless of where GitHub points pes10k.github.io to.