Hardware, or computer hardware, describes the physical parts of a computer system.
Things like the CPU (central processing unit), RAM (random access memory), motherboard, and case are all considered hardware. Computer hardware can also include things that are not required for the computer to function, like a monitor and mouse.
The term hardware is often contrasted with software.
Software refers to collections of instructions and/or data that run on hardware. Software is "soft" in the sense that it can be rewritten.
Computer hardware is "hard" because it's physical, and is difficult to change once it's been produced.