Fn is a hotkey for second function. For your laptop series on F5 key second function should be "Screen blank" - Turns the display screen. Press any key to return. Icon on it looks like two squares - left one is white with sun in it, then arrow to the right one, and that square is dark colored.
When you press Fn + F5 it activated this function "Screen blank".
Pressing any other key deactivates blank screen.
Pressing Fn without second key does not do anything.