Android is basically LINUX. An open source UNIX like operating system. "root" is the most privileged user in LINUX. "Rooting" means you have complete control over the device down to hardware and software settings. Phones are typically not sold this way by the manufacturers because they can't be burdened with people who do not know what they are doing "Bricking" their phones.
Another term for your lexicon - Bricking. Making the device completely unusable and unfixable.