The root directory, or root folder, is the top-level directory of a file system. The directory structure can be visually represented as an upside-down tree, so the term "root" represents the top level. All other directories within a volume are "branches" or subdirectories of the root directory.
While all file systems have a root directory, it may be labeled differently depending on the operating system. For example, in Windows, the default root directory is C:\. On Unix systems and in OS X, the root directory is typically labeled simply / (a single forward slash). As you move up directories within a file system, you will eventually reach the root directory.
Website Root Directory
The root directory of a website is not the top-level directory of the volume, but rather the top-level directory of the website folder. On an Apache server, for example, the name of the root directory is public_html and is located within primary user folder for the website. Therefore, the root folder of a website can be accessed using a single forward slash. For example, a link to a file called home.html within the root directory of a website can be accessed using the reference "/home.html."