Since .rar is a compressed format, you must extract the files before they can be used:
Use a utility like 7-Zip or WinRAR to right-click the file and select "Extract Here".
In competitive gaming (like FPS titles), these packs are often shared to help users maximize frame rates (FPS) by lowering demanding graphical settings or optimizing engine parameters.
Most packs require you to move the extracted files into a specific directory, such as a game's \cfg\ or \mods\ folder.