Having root-level access to a server signifies that you can view and change any file on it, which includes essential system files. You can also install software that can change certain settings on the server throughout the, so that the installation can be done only if you are signed in as the root user. The aforementioned has full privileges, i.e. you will have full control over your machine whenever you want and you can do anything you need. You can also create additional users either with identical rights or with fewer rights, depending on what you need 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 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 to the server and to manage it at the root level.