A subdirectory is a directory below another, called a parent directory, within a data storage system.
In an operating system with a GUI, a subdirectory would be a folder within a folder.
A subdirectory in website development and hosting would typically be a web page leading to other web pages. Examples of subdirectories within a website are shops, help and faq pages, blogs and knowledge base pages, etc. A website subdirectory can be identified by looking for the suffix after the domain, for example, www.examplewebsite.com/blog/.