Even though Joomla is a great platform as it is, there are times when you will have to alter the core files to fit certain needs. For instance, you can't stand how the elements in Joomla content are put together in Joomla. Or your client wants to alter the way the date is shown, where the author appears, how the list of contacts is displayed. We could go on for ever.
The point is: You don't want to be restricted by the platform, do you? You won't have to!
I will show you the basics of how to customize the output from Joomla components, modules and plugins, by applying something called template overrides. Sounds scary? Relax, they're your friends.