Having root-level access to a server means that you'll be able to view and modify any file on it, which includes crucial system files. You may also set up software that can change particular settings on your server during the process, so that the installation is possible only if you're logged in as the root user. The latter has full privileges, i.e. you will have full control over your machine at any time and you'll be able to perform anything you'd like. You may also make other users either with similar rights or with fewer rights, according to what you want them for - everyday tasks, other server admins, etcetera. To be on the safe side, it's generally recommended to use the root account only when you actually need it and not for tasks that you can do through other accounts. You can use a web-based graphical interface or a Secure Shell console to be able to connect to the server and to control it at the root level.