An example of loose coupling vs tight coupling please?

Sure, let me give it a shot. Won't be any code, though.

Django is a Python web framework that comes with auth built in, so you can create users, let them log in, etc. For years, Django's User could only be the one that Django came with. Everything expected the class to live in a certain place and look a certain way. If you wanted some special new data stored about your user, you had to add a new class (since these classes tie into the database, they're called "models" and that's what I'm gonna use from here on) and then associate that model with your user model.

That's really tightly coupled. I can't, for instance, make a user that doesn't have a username, only an email address. Or change the rules for their passwords.

Eventually Django fixed this by loosening the coupling of the user model. It still includes the default one, but by changing a setting in your project you can say that some other model should be used for creating and validating users. The authentication system is no longer tightly coupled to the User model.