How do interfaces achieve loose coupling?

Tight coupling is the situation in which a change in a module forces new changes to be performed in other modules. on the other hand Loosely coupled architecture means changes in one module / section / component hardly affect the other components and each module is somewhat independent of each other. Technologically independent , Build independent and may be even release independent. Loosely coupled architecture is robust ( as problem doesn't propagate to related application ) and easy to maintain and scale. Example are - SOA ( Service Oriented Architecture ), MVC ( Model View Controller ).

     Interfaces are used for “late binding” ie not knowing exactly what type of object will be used. One of the best example of interfaces are sim cards.  We have a number of service providers for cell phone service and a number of handset models. The handset models do not know the internal working of the service providers but are still able to connect to them as long as they have a compatible sim card. Here the sim card is the interface between the handset and the service provider. The handset manufacturer does not need to know how the service providers internal workings all they need to do is call the functionality within the sim card.

In interface, you declare the basic structure or skeleton of a class. That is why it is implemented by another class and not extended. Whatever you declare inside interface must be public static final. So, all the functions have to be defined by the class implementing it. If you want a structure to be implemented by many classes declare them in an Interface and carry on.
Rest all, may it be Abstract class or normal parent class, you need not define all the functions, etc. in the child class. So, basically Interface gives you a skeleton of how the other classes using it must look like.