Hi there
HID stands for Human Interface Device, it is a class in USB protocol which define hardwares like keyboards, mouse, printers, etc. or any human-computer interface device which uses USB as a medium of communication. To be specific, it define all the functions related to such hardware devices which help them to work like they do. For example, how does the computer comes to know which key have you pressed on the keyboard ? It is this class which decodes it and returns the result to computer. It is just one of the many things a HID class does.
Hope this was helpful 