Conversion Rate Optimization (CRO), as defined on Wikipedia, "is the method of creating an experience for a website or landing page visitor with the goal of increasing the percentage of visitors that convert into customers."
Imagine that it's a few months before the Presidential election. You've just received a call, asking if you'd be willing to offer your services as a usability consultant on a political software project. You're curious; what would something called "Orca" be for? You decide to do it.
Front Door Disease is defined by an organization that focuses all of their design and conversion goals on the front page of their site -- ignoring sub pages. A conversion goal can be a donation, email sign-up, or a new user registration.
If you drop a user on the front page of your site they will do one of two things: find desirable content or abandon hope and leave your site. It's your goal, as a site owner, to present clear navigation paths.