What does "WINAPI" in main function mean?
This is a macro definition intended to denote the Windows calling convention. From MSDN:
The way the name is decorated depends on the language and how the compiler is instructed to make the function available, that is, the calling convention. The standard inter-process calling convention for Windows used by DLLs is known as the WinAPI convention. It is defined in Windows header files as WINAPI, which is in turn defined using the Win32 declarator __stdcall.