So you set up your Joomla! site, made it live, and later realised that you'd forgotten to enable the HTAccess file for SEF URL's.
The end result being that all your URLs contain /index.php/ What to do?
You could just enable the HTAccess file, but all the old URL's will then return a 404. Not great if your site has already been indexed by search engines, even worse if others have linked to you too.
In fact, on newer sites, it might even be worse - the old link will still be valid, but there'll be a 'new' link too, so you'll end up with two URLs for the same content.
It's actually incredibly simple to resolve, and this documentation details the two steps you need to take to resolve it, without breaking the old URLs.