c++ : What does extern mean in a function declaration?

What does extern mean in a function declaration?
Using extern in a function declaration we can make a function such that it can used outside the file in which it is defined. 

An extern variable, function definition, or declaration also makes the described variable or function usable by the succeeding part of the current source file. This declaration does not replace the definition. The declaration is used to describe the variable that is externally defined. 

If a declaration for an identifier already exists at file scope, any extern declaration of the same identifier found within a block refers to that same object. If no other declaration for the identifier exists at file scope, the identifier has external linkage.

What can I safely assume about the initial values of variables which are not explicitly initialized? 
It depends on complier which may assign any garbage value to a variable if it is not initialized.