That line of code defines a pure virtual function in C++. It has nothing to do with the otherwise tricky Win32 API or GUI code in general.
A pure virtual function is a virtual function that is used when the designer of the class wants to force derived classes to override the function and provide their own implementation.
If a class contains any pure virtual functions, it is considered an “abstract” class and instances of that class cannot be instantiated.
C++ uses the special syntax = 0; to indicate pure virtual functions instead of adding a new keyword to the language (as languages like C# do). You can think of it as setting the function pointer to 0.
Also see the answers to this related question: What are the uses of pure virtual functions in C++?
(By the way, the Windows header files <windows.h> simply define NULL as 0. So the programmer technically could have written = NULL, but it’s much clearer to use the numeric constant 0 and reserve NULL for pointer values.)