Hmm... never heard of them. I just searched the project that inspired the question you linked to, and there's nothing in there with a "DRF" extension. Checking here doesn't turn up anything Delphi-related. But the fact that it's a linker error, not a compiler error, would lead me to guess that the first two letters stand for "Delphi Resource."
Try a search through your project's directory tree and see if you can find anything with a DRF extension. If so, try opening it with a text editor to see if it's readable, and if not, try a hex editor if you know anything about reading binary file formats. See if you can make any sense of it.
If you don't find any, then Delphi's probably getting it from somewhere in the code it's compiling. Try running a grep search for "DRF" on your directory tree and see if it turns up anything.