Declared in SWI-Prolog.h as follows:

typedef foreign_t (*pl_function_t)();

PL_EXPORT(int) PL_register_foreign(const char *name,
                                   int arity,
                                   pl_function_t func,
                                   int flags, ...);

and defined in pl-fli.c although with a different argument list:

int
PL_register_foreign(const char *name,
                    int arity,
                    Func f,
                    int flags, ...)

In shared libraries, this function is called from a specially designated C function that is called on module load so that the predicates implemented implemented by the module become visible. See: https://eu.swi-prolog.org/pldoc/man?section=foreignlink