Trouble in converting C code to D

[Suliman via Digitalmars-d-learn]

Oh in gdal.d there is comment:

  /*
   * The enum CPLErr is defined in the header file cpl_error.h. I 
have not
   * for the time being included a binding to that header, but 
have just
   * imported this single symbol from it.
   *
   * Similarly, GDALProgressFunc is defined in port/cpl_progress.h
   */

Am I right understand that binding is not file with .d extension? 
So what is this file with extern(C) and so on. It's named 
interface file?