Define Interface Simplicity vs Implementation Simplicity

In Richard Gabriel's The Rise of ``Worse is Better'', he talks about the simplicity of interface vs the simplicity of implementation in a design.

I've never been able to quite grasp what he means and searching the web for the answer has only yielded information about Java's interfaces (the structure of classes, etc) vs the code in Java's methods and classes.

What does he mean by interface and implementation and what does simplicity mean for each of them?