The archive contains a save of the finished game so you can replay any level, with some extra cheats and outfits unlocked.
Important: make sure you backup (copy) your existing savegame directory before unpacking, you will LOSE your own progress.
Attention: the savegames might not be compatible with all versions of the game.
Download the archive with the savegames and unzip it (you can use a program such as WinRAR or Windows XP/Vista function). Then you need to go to My Documents folder where you should see another folder called Eidos.
Open it and inside you should see a directory called Tomb Raider - Anniversary. Inside this last folder you should see Default Profile and other profiles you might have. Backup (copy to another location) the existing Default Profile then delete it..
Copy the directory you extracted from the archive so you see a new Default Profile folder.
Make sure that is the exact directory and you didn't unpack it into a folder with some other name. To make sure, enter the directory, you should be able to see a list of files with the extension .TRASave. Then you are ready to play the game.