"Rooting" means exploiting a security flaw to get root privileges in a device that is designed to prevent users from doing that (e.g. the iPhone or the Android phones sold by some US network operators).
Bootloader unlocking and root access was available and well documented on the first Android device designed by Google (the Nexus One), simply by running the command "fastboot oem unlock".
The same command worked on the second Android phone by Google, the Nexus S, and all subsequent devices, including tablets: Galaxy Nexus, Nexus 4, Nexus 7 and Nexus 10. Unsurprisingly it also works on Glass.
It's just a well know feature of all the devices created by Google. Details: https://plus.google.com/112413860260589530492/posts/jYHhKHYwUJ2.
Google's documentation on how to "root" your Android devices: http://source.android.com/source/building-devices.html.