Every F[0-9]+ key is two things on the Mac. It's a function key, with its definition set by a carefully negotiated contract between the OS and the current application, or it's a non-changeable controller with a pretty picture (volume, screen brightness, etc.)

You do one by pressing the key, and the other by pressing it with the Fn modifier key. Which one is default depends on a pref setting in Keyboard & Mouse > Keyboard Shortcuts.

Personally, I set it to use the F[0-9]+ keys as function keys, and use the Fn modifier to access the volume etc.

This is a default setting. I believe you should be able to modify it on System Preferences>Keyboard and Mouse>Keyboard : Use F1, F2 as standard keys.