Having root-level access to a server means that you can view and change any file on it, including important system files. You can also install software that can modify certain settings on your server throughout the, so that the installation can be performed only if you're signed in as the root user. The latter has full privileges, i.e. you'll have full control over the machine whenever you want and you'll be able to perform anything you want. You can also make various other users either with similar rights or with fewer rights, based on what you need them for - daily tasks, other server admins, etc. To be on the safe side, it is usually recommended to use the root account only when you actually need it and not for tasks that can be done 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.
