A document root is a file system directory, where Apache (the web server) reads files from to serve contents for a website. The document root itself (typically) translates to the top level website URL, and subdirectories created under the document root translate to directories of a URL.

For instance, your primary virtual server would have an URL of http://domain.com/ and a document root at ~/web/public, whereas a part of the website on your secondary virtual server with the URL of http://othersite.net/resources/ would read from a directory of ~/domains/othersite.net/web/public/resources.

In general, document root is always the web/public directory under the home directory of the particular virtual server.