A subdirectory is a folder that exists within another folder on a computer or server. It helps organize files by creating a hierarchy, making it easier to manage and locate documents. For example, a main directory called Documents might contain subdirectories for Work, Personal, and Projects. Subdirectories are commonly used in web development as well. A website's structure often includes subdirectories to categorize content, such as example.com/blog or example.com/images. This organization not only aids in navigation but also improves search engine optimization (SEO) by providing clear pathways for search engines to index the site's content.