fn + function key enables you to use the F1 to F12 keys on a laptop. If you don't press it the keys just do the function that the picture relates too (volume control and brightness are a couple if I remember correctly).
if you're on a laptop, you'll see a lot of keys with subtext. for instance, the up arrow has "page" written on it, so pressing "fn" + "up arrow" will be the equivalent of pressing "page up."
fn + function key enables you to use the F1 to F12 keys on a laptop. If you don't press it the keys just do the function that the picture relates too (volume control and brightness are a couple if I remember correctly).