A root is defined in the computer world as the top-level directory of a file system. Top-level directory means that all the other directories - including subdirectories and the files they contain - are included. The root directory is designated by a forward slash ("/") or backward slash ("\") depending on the system in question.
Techopedia explains Root
The term "root" was adapted from a tree root because this data structure looks like an upside-down tree. The folders in the tree structure are the branches and the files themselves represent leaves. Because it is an upside down tree structure, the root is shown at the top of the system directories with all the others spanning down and outward from it.
In a Windows-based system, "C:\" depicts the root directory of the C drive. However, on Macintosh and Unix based systems, a simple forward slash depicts the root directory. Root directories are not limited to operating systems; they are also used in URL addressing, which shows the domain name as the top level or root. This is followed by a forward slash to denote different pages or directories.