Its for access to the "f" keys, for example, press the num lock / f6 key and the num lock light comes on, hold fn and press the same key, and it'll activate the f6 key...If that makes sense.
Just a helpful hint.. If you press fn + f9 you get expose, fn + f10 you get a local expose and you can just press f11 to hide all and f12 for dashboard.