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 app regardless of the changes that it will make to the software environment on the server, and you'll 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 all these things and also to create other users, which includes users who also have full privileges. This 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 have to customize the server or to install software that may be required by some script-driven app that you'd like to run.