Having root-level access to a server means that you can view and modify any file on it, including important system files. You may also install software that can update specific settings on the server throughout the, so that the installation can be done only if you're logged in as the root user. The aforementioned has full privileges, i.e. you'll have full control over the machine at any time and you can perform anything you need. You may also create additional users either with similar rights or with fewer rights, based on what you require them for - everyday tasks, other server admins, etcetera. To be on the safe side, it is usually recommended to use the root account only when you really need it and not for tasks that you can do using other accounts. You may use a web-based graphical interface or a Secure Shell console to connect to the server and to control it at the root level.