what does fn button mean
When you often use a function from the menu bar often, the shortcut is usually next to it. Ok, then: fn (function) is meant for the function keys f1-f12 (or more on larger keyboards). It's pure meaning is to switch between the usual f-behaviour and OS X functions (media control, brightness, etc.). It's also used for controls that are potential standard-keys but don't fit on the smaller keyboards (a larger [USB] keyboard from apple for example has a delete key, to save space the smaller keyboard put's it on a fn-function).