Having root-level access to a server means that you can view and change any file on it, including crucial system files. You may also install software that can modify specific settings on the server throughout the, so 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 the machine at any time and you can perform anything you would like. You can also make various other users either with the same rights or with fewer rights, according to what you need them for - daily tasks, other server administrators, etc. 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 via other accounts. You can use a web-based graphical interface or a Secure Shell console in order to connect to the server and to manage it at the root level.