Do you have an example of something that you don't understand?
Interfaces are (and I use that loosely) objects. I like to think of them as another way of looking at an object, in terms of what it does. An interface describes the behaviour contract of an underlying object. Any object that can fill this contract can be used.
Example is ResultSet; you know that there is an object there and that you can call the next() method to get a result. The only thing you don't know is the name of the class that is responsible for providing the result returned by next().
Everything is theoretically impossible, until it is done. ~Robert A. Heinlein