Answer Description
Many laptops use the Fn Key as a modifier key similar to Ctrl, Shift and Alt keys. Fn key usage varies, but common examples are to re-use number keys for additional functionality like changing a device's volume.
Wikipedia
The Fn key, short form for function, is a modifier key on many keyboards, especially on laptops, used in a compact layout to combine keys which are usually kept separate It is typically found on laptops due to their keyboard size restrictions It is also found on many full-sized "multimedia" keyboards as the F-Lock key It is mainly for the purpose of changing display or audio settings quickly, such as brightness, contrast, or volume, and is held down in conjunction with the appropriate key to change the settings
Fn_key - Wikipedia, the free encyclopedia