IW4x allows you to customize your game to your own personal liking, this can be anything from changing the main menu music to replacing camo textures and much much more. You do this by telling the game to load your new custom assets (gsc scripts, images, sounds, menu files,..) instead of the base game ones by using the userraw folder. I will now explain the basics to enable you to do so:
The game assets are contained in archives called .iwd files, these are zip files and can be opened by programs such as WinRar. The location of these files are in the "Main folder" and "iw4x" folder both located in the root MW2 folder. Example: iw_00.iwd, iw_01.iwd, ..., iw_23.iwd and so on. If you open any of these with a program such as WinRar or similar program you will see the games assets and folder structure they follow. The game pulls all the assets contained in each of these archives into one large pool of assets on launch. Imagine they extract all the iwd files content into one virtual folder and that's where the game pulls all it's assets from.
The userraw folder is used to override any asset contained within the main .iwd files, on launching the game, the game will use assets found in userraw over what is in the main iwd file. So any textures/sounds/scripts/... you place in userraw will be used over the ones found in main. The importance of this is that it avoids you editing the basegames .iwd files, any edits to these would work fine but as soon as an update comes out or you repair your game the updater.exe will delete any changes and restore it to it's original state. So userraw is the cleaner solution.
Assets can either be placed as loose files/folders (replicating the folder structure inside of the iwd file they came from) in userraw or can be packed into .iwd files like how they are in the main folder. The choice is yours however it may be more clean to .iwd files but may be easier for beginners to use loose files.
Simply place the assets you wish overwrite the base game with, ensuring you replicated the folder structure in the iwd file. As an example I will explain the process of replacing the main menu music.
Loading up the game now will result in your custom mp3 file in userraw playing instead of the normal main menu music.
The same rules apply as above but this time you only need to place the downloaded files and extract them into userraw or if they come in an .iwd file simply place the .iwd inside of userraw "userraw\z_DownloadedFile.iwd"