DRM "Stands for "Digital Rights Management."
It's technology that prohibits you from sharing and copying things you buy from a digital marketplace, such as Apple iBookstore and other companies that sell devices that support eBooks.
Backers who will be getting a copy of Camera Ready will get it without DRM, meaning there are none of the aforementioned restrictions. If funding is successful, we'll contact backers via Kickstarter survey and ask them what kind of device they have (iPad/iPhone, Kindle Fire, or Barnes & Noble Nook). You will then get a link to download Camera Ready in the appropriate format. Bottom line — it's the best way to get you your reward without having you go through too much red tape!
You can read more about DRM in eBooks on Wikipedia: http://en.wikipedia.org/wiki/Digital_rights_management#E-books