Having root-level access to a server means that you will be able to view and update any file on it, which includes essential system files. You may also install software which can modify specific settings on your server during the process, so the installation is possible only if you're signed 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 do anything you would like. You can also create various other users either with the same rights or with fewer rights, depending on what you require them for - daily tasks, other server administrators, etcetera. To be on the safe side, it's usually recommended to employ the root account only when you really need it and not for tasks which can be done via other accounts. You can use a web-based graphical interface or a Secure Shell console to be able to connect with the server and to control it at the root level.