Res Files
Res files are resource files. Basically by using a res file you are telling half-life about files that are needed for your map that the program is not expecting.
The res file is named the same as your map and has a .res file extension on it. So for example if your map is mymap.bsp, then your res file will be named mymap.res.
Res files should be placed in the svencoop/maps directory.
You create a res file with a text editor such as Notepad. If you use Notepad be careful to put a .res file extension on the file before you save it. Then in the "save as type" area choose all files. If you just save it without doing that, some versions of Notepad will save it as a .txt file.