If you have root-level access to a hosting server, it means that you'll have full control over it. That is to say, you can set up any server-side application no matter what the changes that it'll make to the software environment on your server, and you will be able to both access and modify any file, which includes system files, and change any settings. You can do this by connecting to the server as a root user that has full privileges to perform all these things and to generate other users, which includes users that also have full privileges. The connection can be established with a web-based graphical interface or a Secure Shell console. For safety reasons, it's recommended that you create a separate user for your everyday work and use the root user only when you need to customize the server or to install software that may be needed by some script-driven application that you wish to run.