An input device is a piece of hardware that is used to enter data into a computer.
There are many different kinds of input devices. They are split into two categories - manual input devices and automatic input devices.
You don't need to know a great deal about any particular one but you do need to be able to make suggestions as to the most appropriate input device to use in a given scenario. You also need to be able to explain why your choice is the right one and compare it to other input devices that you could have chosen.