Fn Lock is a special function key that is typically found on laptop keyboards. When Fn Lock is enabled, the F1 through F12 keys will perform their secondary functions, which are typically listed on the keycaps. For example, the F1 key might be used to mute the microphone, or the F2 key might be used to decrease the screen brightness.
Fn Lock can be useful if you frequently use the secondary functions of the F keys. However, it can also be annoying if you accidentally press Fn Lock and then forget to turn it off. If you find yourself accidentally pressing Fn Lock frequently, you may want to disable it.