Basically so that you can set all those SEO settings in the global configuration tab to YES.

Note: you need the permissions to rename the file ("htaccess.txt" to ".htaccess" in your website root folder). You'll need ftp access or a file manager with all your website files in).  Do not continue if you ONLY have control of the Joomla CMS.

Watch the youtube video below which shows you how to do it