HID, short for Human Interface Device, is a standard USB device class that covers keyboards, mice, trackpads, game controllers and similar input devices. Because HID is a well defined, generic standard, an operating system can recognise and use these devices immediately, without needing a manufacturer specific driver.
This is why a basic USB keyboard or mouse works the moment it is plugged into almost any computer, phone or tablet with USB support, while more specialised devices with extra features, such as programmable buttons or custom lighting, may still need additional software to unlock their full functionality beyond the basic HID behaviour.
The HID class is also relevant to security, since the BadUSB attack technique often relies on making a malicious device present itself as an innocent looking HID keyboard to type unwanted commands. See our USB security guide for more on this risk.