Having root-level access to a server signifies that you'll be able to view and modify any file on it, which includes essential system files. You can also set up software which can modify specific settings on the server during the process, 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 at any time and you'll be able to do anything you want. You may also create other users either with identical rights or with fewer rights, depending on what you require them for - day-to-day tasks, other server administrators, etc. To be on the safe side, it's usually recommended to use the root account only when you actually need it and not for tasks which can be done using 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.