A Bluetooth profile (mode) for remote control of one device from another, using human-interface hardware such as a keyboard, keypad, joystick, d-pad, etc.
Not all Bluetooth devices support HID, and many phones with HID only support control in one direction.
An example scenario might be a text keyboard accessory used to enter text on a mobile device, linked wirelessly by Bluetooth and HID.
However, many phones with HID only support control in the reverse direction from the example above. That means you could use the phone's joystick and keypad to control another device via Bluetooth, such as a toy car or a TV set-top box.