In C++, this indicates that a function uses the C calling convention, et cetera, rather than those of C++.

Can be used like this:
extern "C" {
   void f( int somearg );
   void g( int somearg, char *somestring );
}

or:
extern "C" void f( int somearg );
extern "C" void g( int somearg, char *somestring );

Often #ifdef __cplusplus is done in a header file to do an extern "C" if the compiler is a C++ one.