The directory (in the real filesystem) from which your Web server will be serving most of its Web pages.
For example: If you set the document root to
/home/httpd/html, then accesses to
http://your.webserver.com/index.html would return the file
/home/httpd/html/index.html. An access to
http://your.webserver.com/foo/gazonk.gif would
return /home/httpd/html/foo/gazonk.gif.