An interface in C++ is a class with only pure virtual functions and no data members. It defines a contract without any implementation. Other languages have a dedicated interface keyword, but C++ uses abstract classes.
You might define interface Drawable with pure virtual draw() and resize(). Any class implementing this interface must provide both methods, but the interface itself has no code. Interfaces let unrelated classes share capabilities.
A Window and a Shape have nothing in common except both can be drawn. They each implement Drawable independently.