Storage space on disc is divided into directories (Windows calls them folders).

Every disc, when formatted,  has a root directory.

When we save work to   C:\   we are saving it in the root directory of drive C.

You can create directories inside this root directory. For example you could create a  directory called docs inside the root directory. To save work in this you would save it in   C:\docs

A directory created inside another directory is called a subdirectory.

You can have subdirectories of subdirectories etc....building a hierarchical structure of directories on the disc.

The Internet also uses a hierarchical structure of directories on the discs of its servers. For example..

www.myserver.co.uk/members/buster/index.htm

refers to a file called index.htm which is stored in the directory buster which is a subdirectory of the directory members stored on the server www.myserver.com.