The .res file is a file on the server which lists the custom files (sounds, textures, etc) that need to be downloaded for the client for a particular map.
The .res file is placed in the maps folder on the server. It has the same name as the map file but with .res on the end, for example:
map file: my_room.bsp
res file: my_room.res
Here is a good tutorial on the subject:
