Mirage OS is pretty damn cool. The way I'd describe it is like this: Imagine pulling the operating system out from under your application and all the layers drop down a level.
More specifically, the idea is to run normally application-level code (like web servers, databases, etc.) at the operating system level, on top of nothing other than the Zen Hypervisor.
It's kind of hard to explain, and until you understand it, it's hard to see the advantage, but once you get it, it's pretty amazing stuff.
I also recommend following along with the blog, it makes some things more clear by presenting them in a more application-centric context:
If you want to just jump straight into the deep end, this paper explains more abstractly the motivation for the project and the cases/applications where it makes the most sense: