The default behavior of Mac function keys is to perform the action indicated by the icon printed on the key. In this case, the action performed will vary depending on (a) the application you are currently using or (b) the keyboard shortcuts listed in the Keyboard & Mouse pane of System Preferences.

(1) to perform special actions that correspond to the icon printed on the key, such as dimming or brightening the screen, showing the Dashboard, increasing or decreasing speaker volume, and so on. On a Mac, function keys can be used in two ways: These keys (F1 - F12) are called function keys. Modern Mac computers using an Apple keyboard have icons printed on some of the keys on the top row of the keyboard. This is especially important with Excel, which uses a number of function keys for shortcuts. If you want to master Excel keyboard shortcuts on a Mac, you need to take a moment to understand how the Mac keyboard is arranged, and how it can be configured through system preferences.