Windows 3.x provides functions for memory and file manipulation using far pointers, which have the form _fxxxx. In Win32, these functions are replaced by similarly-named functions of the form xxxx, because there is no need for far pointers in Win32. (The _f prefix is dropped from the name.)
The WINDOWSX.H file defines the _fxxxx function names so that in Win32, the _fxxxx function names are equated to corresponding functions that are still supported. This means that as long as you include WINDOWSX.H, you don’t have to rewrite calls to these functions. Some of the definitions are:
#define _fmemcpy memcpy
#define _fstrcpy strcpy
#define _fstrcmp strcmp
#define _fstrcat strcat