If you have root-level access to a hosting server, it means that you'll have full control of it. That is to say, you'll be able install any server-side application regardless of the changes that it'll make to the software setting on your server, and you'll be able to both access and change any file, including system files, and modify any settings. You can do this by connecting to the server as a root user which has full privileges to do the aforementioned things and to make other users, including users that also have full privileges. The connection can be established through a web-based graphical interface or a Secure Shell console. For safety reasons, it is recommended that you create a separate user for your everyday work and employ the root user only when you need to customize the server or to set up software that may be needed by some script-driven app that you want to run.