[dbul]

"Secure Boot is designed to address the potential for malware to insert itself between the firmware and the operating system on your computer. It accomplishes this by enforcing that only “approved” software is able to boot in your computer by way of a key that recognises pre-approved and signed software."

That's from the linked canonical article on the issue: http://blog.canonical.com/2011/10/28/white-paper-secure-boot...