What do we mean by interface in the definition of Facade Design Pattern

I was studying design patterns and while I was studying Facade Design Pattern, I saw an explanation in the book as:

A Facade Pattern says that just "just provide a unified and simplified interface to a set of interfaces in a subsystem, therefore it hides the complexities of the subsystem from the client".

Yet when I analyze the implementation of Facade Design Pattern, what I see is that every method that is used inside of the class that is used for Facade Pattern is used as concrete classes.

What do we mean by set of interface in the explanation? Do we mean that we could use some interfaces for the concrete classes or is it something like we use with adapter which is the logic that we try to adapt different interfaces to use simplified versions of them?