The ACPI Function Key Handler is a Windows driver associated with the ACPI (Advanced Configuration and Power Interface) system, which is a standard for power management and configuration of devices on a computer. The ACPI Function Key Handler specifically enables the functionality of special keys on the keyboard, such as those used for adjusting the volume, brightness, or other special functions (like media controls or power management) on laptops or desktops.

In simpler terms, it allows the function keys (such as F1-F12) to be properly mapped and respond to specific actions like adjusting screen brightness, enabling or disabling wireless, muting the microphone, etc., depending on the manufacturer of your laptop or keyboard.