The DirectoryIndex directive is used when you want to change
the default page that is displayed when someone accesses
http://www.your-domain.com/
from
index.html
to something else.

For example If you wanted to have
my-page.html
as default you would add the following line in the .htaccess file that
resides in your main public directory.
DirectoryIndex mypage.html
The result would be that every time someone accesses http://www.your-domain.com
the page that is displayed is http://www.your-domain.com/my-page.html
Of course you can use a php or htm page
DirectoryIndex my-htm-page.htm
DirectoryIndex my-php-page.php
and more important you can use variables in the php page as in the following example
DirectoryIndex my-php-page.php?artist=Nicole-Kea&group=PCD