It allows your notebook to perform extra functions without having additional keys. You'll see a number of symbols marked on your keys (generally in a different color than the letters on the keys. The symbols' colors should match the color of the text on the Fn key) - the sun will shift your screen brightness up and down, the speaker will change your volume - there are lots of different functions that your notebook can do. Just hold the Fn key and then hit the key with the symbol on it you want. Consult your manual for a full list of what your specific notebook can do.