Having root-level access to a server signifies that you can view and modify any file on it, including crucial system files. You may also set up software which can modify particular settings on the server throughout the, so the installation is possible only if you are signed in as the root user. The aforementioned has full privileges, which means that you'll have full control over the machine whenever you want and you'll be able to perform anything you want. You may also generate other users either with similar rights or with fewer rights, based on what you require them for - day-to-day 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 you can do via other accounts. You may use a web-based graphical interface or a Secure Shell console to connect with the server and to control it at the root level.