Extern keyword in C? What does extern keyword do? examples please?

extern
Allows one module of your program to access a global variable or function declared in another module of your program.
You usually have extern variables declared in header files.
If you don't want a program to access your variables or functions, you use static which tells the compiler that this variable or function cannot be used outside of this module.

errno.h
#ifndef ERRORS /* prevent multiple inclusion */
#define ERRORS 0
extern int errno; <- Declaring the variable to be external.
extern void seterrorcode(int errcode);