Human Interface Device is one of the features which most of the OS uses to consider peripheral devices to be used by the machine, in simple language when a device is HID-compliant it is always treated as an input device by Operating System. This feature can be found on every OS, in windows, it can be found in the Device Manager section where the input devices such as ass keyboard and mouse will be shown as HID devices.
This HID feature can be exploited very easily if any input device can be made HID complaint, with the help of software integrations HID can be interfaced with cheap microcontrollers such as Arduino, STM32 blue etc. MSP430 is a microcontroller from Texas Instruments which has an HID ready feature which allows it to act as an input device recognised by the system. Hence just with keystrokes, it can do tasks in the OS as any real Key has been pressed.