When you have root-level access to a hosting server, it means that you'll have full control over it. In other words, you'll be able install any server-side app whatever the changes that it'll make to the software environment on the server, and you are able to both access and change any file, which includes system files, and modify any settings. You can do this by connecting to the server as a root user which has full privileges to perform all these things and to make other users, including users that also have full privileges. This connection can be established through a web-based graphical interface or a Secure Shell console. For safety reasons, it is recommended that you make a different user for your everyday work and employ the root user only when you have to modify the server or to set up software which may be required by a script-driven application that you wish to run.