This device is the reincarnation of a previous Nintendo product, the Super Game Boy (which allowed you to play your GB games on the Super NES), except this time the system does more than just allow you to plug the cartridge into the console. The Game Boy Player also sports a Link Cable Port, allowing you to hook up several Game Boy Advances as if it were the handheld itself.
The Game Boy Player simply plugs into the bottom of your GC and after applying a couple of screws and slamming the GB Player Startup Disc, you are on your way to playing all of the games in your Game Boy library (that is, everything from the original Game Boy all the way to the Advance line).
With a GB game running through the Player, you will be able to use the GameCube Controller to move your characters around, and the buttons not found in your typical GB controller scheme (like the X, Y and Z buttons) are used to change the settings in your Player.