HID stands for "Human Interface Device". Keyboards, mice, digitizer tablets, joysticks, and game controllers are all HID devices. CircuitPython can emulate three standard HID devices by default: mouse, keyboard and consumer control. These are described in more detail in CircuitPython Essentials Guide and the Customizing USB Devices Guide.
All operating systems (e.g., Windows, macOS, or Linux) have built-in support for certain standard devices like keyboards and mice. But they vary about whether they support other less common devices. Often you need to install a driver to support a particular HID device, such as a specific game controller or a tablet.
There are so many kinds of devices you might want to implement in CircuitPython, it cannot provide built-in support for them all. So CircuitPython lets you define and implement your own custom HID devices . With the right device definition and built-in OS support or a compatible driver, you can implement an existing HID device, or define your own custom variation.