Fn (Function) Button

Nikon D5100 fn (function) buttonExplanation: The Fn button is a programmable function key that can get one out of several possible functions in the custom setting menu f1 Assign Fn button.

The symbol on it represents its default setting, the activation of the self timer.

It is designed for settings that you want to have available immediately, without scrolling through menus or the shooting information.

Recommendation: I prefer to use it for white balance, see the recommendation for the custom setting menu f1 Assign Fn button.