root is the superuser in Linux operating systems and since Android is also a Linux based operating system, it is the superuser in Android. Superuser means that it has complete control over the system files, memory, etc. root user allows you to make changes that are otherwise denied using the normal (regular) user. This is the way you should use your computer (well, it applies to all devices as it is a good security practice) and only use root when you need to make changes to your system to prevent accidental damages to your system. root in the context of Kindle Fire also means that you are stuck with Amazon's Apps and way of accomplishing tasks. some people are satisfied with the default stock and that's okay, not everyone feels adventurous to try rooting & custom roms.