Not every keyboard has one, but if yours has a key marked Fn, F Lock or Fn Lock, it might not immediately be clear what it does. Pressing one of these keys alone does nothing. At most, depending on your configuration, it might turn the Fn Lock key indicator on. Even more confusingly, you may have an Fn key and see Fn Lock or a lock icon printed on a different key with no indication of what either does.
The Fn and Fn Lock keys are most often found on laptops, but they appear on some standalone keyboards as well, especially smaller ones, like Logitech's G512 X. They're used with the function keys (the F1 to F12 keys on the top row) to add support for more controls to compact keyboards or, in the case of Mac users, activate Google's Gemini AI. Their exact functions and how you turn them on and off vary depending on your keyboard but there are a few basic ways they work.
How to use Fn Lock and change its behavior
You can think of the Fn key similarly to the Shift key. When you want to activate it, you need to keep it held down the whole time you're using it. While it's held, it will switch between the function keys' standard actions and an alternate set, usually printed on or above the key itself. Whether the function keys' normal behavior or the alternate actions are the default depends on the keyboard, but usually alternate actions like media controls are the default.
In comparison, Fn Lock works like the Caps Lock key, turning the function keys' alternate mode on with one press and off with another. It's much less common to have a dedicated Fn Lock key than an Fn key but they still appear on some keyboards. Even without a separate lock key, you might still be able to toggle the Fn key instead of holding it. On keyboards without a dedicated Fn Lock key, holding Fn and pressing another key will sometimes perform the same function. It's common for the lock combination to be Fn and Shift or Esc, but look for Fn Lock or the lock icon printed on another key to find it.
If you want to change whether the function key is locked or unlocked by default, you can do that in your BIOS settings. Restart your computer and enter the BIOS, usually by repeatedly pressing an assigned function key or Delete. Names of settings can vary a lot, but if you have support for Fn Lock, you should find an entry called Function Key Mode or Action Key Mode. Enabling or disabling this setting will change whether your keyboard activates standard function key behavior or alternate actions by default, but which setting is which again depends on your specific system. Some laptops or keyboards may also come with configuration software that lets you make the same changes without the trouble of navigating the BIOS settings menu.