What is Res
Res extension stands for Resurce. These source files can be opened by Microsoft Visual c++ software.
It is one of the compilation file which used by Microsoft Visual C++ or Borland Delphi programming languages. Res files include icons,images, mouse pointer, tex tables and software versions which is essential for EXE files that will be generated.
You should use proper programming language to run res files. You have various free options on internet.
Viewer or Converter for Res
Microsoft Visual C++ , Borland Delphi
Which pages about Res