If you actually get one of these in your house, it's probably a sign that you read too much Nancy Drew as a child. And that you have money to burn.
It's a hidden door (from reversica.com) that opens to reveal another room. The hidden door is fashioned like a bookcase, although you can also get it with a section for a tv.
You could spend hours thinking of what to put in a hidden room...people you've kidnapped, S&M equipment, a Faberge egg on a pedestal under glass.
I know what I'd put in mine...more books.