All the physical components of a computer like processor, hard drives, graphics card, memory; this includes personal computers, phones, or any electronic devices. The term is predominately used by enthusiast who build their own computers and frequently swap out parts and upgrade. To get a better idea I would recommend checking out /r/hardware on Reddit to see more examples and discussions.
In the same way you go to Home Depot to buy building supplies or hardware to construct or maintain a house, computer hardware is used to build a computer. A computer case does have quite a few screws in it and you use a screw driver and other tools you could buy at a regular hardware store.