When 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 whatever the changes that it'll make to the software setting on the server, and you will be able to both access and update any kind of file, including system files, and modify various settings. You can do this by connecting to the server as a root user who has full privileges to do the aforementioned things and to make other users, which includes users who 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 make one more user for your daily work and use the root user only when you have to customize the server or to set up software which may be needed by a script-driven application that you'd like to run.