What does the extern keyword mean?

extern gives a name external linkage. This means that the object or function is accessible through this name from other translation units in the program. For functions, this is the default linkage in any case so its usage (in this context) is usually redundant.

The extern keyword declares a variable or function and specifies that it has external linkage (its name is visible from files other than the one in which it's defined). When modifying a variable, extern specifies that the variable has static duration (it is allocated when the program begins and deallocated when the program ends). The variable or function may be defined in another source file, or later in the same file. Declarations of variables and functions at file scope are external by default.

You can find a more complete description here.