Coupling determines how much a class is dependent on another classes. For example we may have the Main class that uses User class (meaning that Main class is coupled or dependent onUser)
If we change the User class, the Main class will be affected and we may need to change it and recompile.
If we had hundreds or thousands of classes, and many of them were highly dependent on each other, we could end up with lots of cascading changes that'd be hard to implement and extend.
A good metaphor would be a car. For a flat tire, we wouldn't need to swap the entire engine. That makes the car a loosely coupled system (different parts work together, but are not highly dependent on each other)
We can build loosely coupled applications using interfaces.