Short for Function, Fn is a key (shown below) found on the bottom left-hand portion of most laptop keyboards. As the name indicates, it is used to perform special functions on a laptop such as turning up the brightness or contrast, switching video modes, or printing a screenshot.
Illustrated usage example
As you can see in the image above, the Fn key is colored green. Keys with symbols of the same color indicate that their secondary function will be activated by pressing them while holding the Fn key down. In this example, you can see that pressing and holding down the Fn key, along with the up or down arrows, would adjust the brightness.
Tip: The Fn key should not be confused with the Function keys F1 - F12 found on all computer keyboards.